| Debt Disclosure [Abstract] | Period [Axis] |
|---|
2016-01-01 - 2016-12-31 |
|---|
Debt Disclosure [Abstract] | |
Schedule of Long-term Debt Instruments [Table Text Block] | The Company’s long-term debt consists of the following as of December 31, 2016 and 2015 (dollars in thousands): | | | | | | | | | | | | 2016 | | 2015 | $300 million 5.625% Senior Notes due 2017 | | $ | 299,945 |
| | $ | 299,671 |
| $400 million 6.45% Senior Notes due 2019 | | 399,805 |
| | 399,737 |
| $400 million 5.00% Senior Notes due 2021 | | 399,025 |
| | 398,803 |
| $400 million 4.70% Senior Notes due 2023 | | 398,986 |
| | 398,835 |
| $400 million 3.95% Senior Notes due 2026 | | 399,985 |
| | — |
| $100 million 4.09% Promissory Note due 2039 | | 94,370 |
| | 96,849 |
| $400 million 6.20% Subordinated Debentures due 2042 | | 400,000 |
| | 400,000 |
| $400 million 5.75% Subordinated Debentures due 2056 | | 400,000 |
| | — |
| $400 million Variable Rate Junior Subordinated Debentures due 2065 | | 318,737 |
| | 318,734 |
| Sub-total | | 3,110,853 |
| | 2,312,629 |
| Unamortized issuance costs | | (22,218 | ) | | (15,081 | ) | Long-term Debt | | $ | 3,088,635 |
| | $ | 2,297,548 |
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Schedule of Maturities of Long-term Debt [Table Text Block] | Future principal payments due on long-term debt, excluding discounts, as of December 31, 2016, were as follows (dollars in thousands): | | | | | | | | | | | | | | | | | | | | | | | | | | Calendar Year | | 2017 | | 2018 | | 2019 | | 2020 | | 2021 | | Thereafter | Long-term debt | $ | 302,582 |
| | $ | 2,690 |
| | $ | 402,802 |
| | $ | 2,919 |
| | $ | 403,040 |
| | $ | 2,000,140 |
|
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Schedule of Line of Credit Facilities [Table Text Block] | The following table provides additional information on the Company’s existing committed credit facilities as of December 31, 2016 and 2015 (dollars in thousands): | | | | | | | | | | | | | | | | | Amount Utilized(1) December 31, | | | Facility Capacity | | Maturity Date | | 2016 | | 2015 | | Basis of Fees | 270,000 |
| | November 2017 | | 270,000 |
| | 270,000 |
| | Fixed | 161,974 (2) |
| | November 2017 | | 31,382 |
| | 66,154 |
| | Fixed | 72,080 (2) |
| | December 2018 | | 72,080 |
| | 36,430 |
| | Fixed | 17,513 (2) |
| | March 2019 | | 17,513 |
| | 45,422 |
| | Fixed | 120,000 |
| | June 2019 | | 85,040 |
| | 85,040 |
| | Fixed | 850,000 |
| | September 2019 | | 96,095 |
| | 313,659 |
| | Senior unsecured long-term debt rating | 100,000 |
| | June 2020 | | 70,690 |
| | 68,657 |
| | Fixed |
| | (1) | Represents issued but undrawn letters of credit. There was no cash borrowed for the periods presented. |
| | (2) | Foreign currency denominated facility, amounts presented are in U.S. dollars. |
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