| Debt Disclosure [Abstract] | Period [Axis] |
|---|
2016-01-01 - 2016-12-31 |
|---|
Debt Disclosure [Abstract] | |
Schedule of Long-term Debt Instruments [Table Text Block] | Long-term debt consisted of the following: | | | | | | | | | | | | | | | | | | December 31, | | Maturity | | 2016 | | 2015 | | | | | | (Thousands of Dollars) | Revolving Credit Agreement | | 2019 | | | $ | 838,992 |
| | $ | 882,664 |
| 4.75% senior notes | | 2022 | | | 250,000 |
| | 250,000 |
| 6.75% senior notes | | 2021 | | | 300,000 |
| | 300,000 |
| 4.80% senior notes | | 2020 | | | 450,000 |
| | 450,000 |
| 7.65% senior notes | | 2018 | | | 350,000 |
| | 350,000 |
| 7.625% subordinated notes | | 2043 | | | 402,500 |
| | 402,500 |
| GoZone Bonds | 2038 | thru | 2041 | | 365,440 |
| | 365,440 |
| Receivables Financing Agreement | | 2018 | | | 58,400 |
| | 53,500 |
| Net fair value adjustments, unamortized discounts and unamortized debt issuance costs | | N/A | | | (968 | ) | | 1,508 |
| Total long-term debt | | | | | $ | 3,014,364 |
| | $ | 3,055,612 |
|
|
Schedule of Maturities of Long-term Debt [Table Text Block] | The long-term debt repayments are due as follows (in thousands): | | | | | 2017 | $ | — |
| 2018 | 408,400 |
| 2019 | 838,992 |
| 2020 | 450,000 |
| 2021 | 300,000 |
| Thereafter | 1,017,940 |
| Total repayments | 3,015,332 |
| Net fair value adjustments, unamortized discounts and unamortized debt issuance costs | (968 | ) | Total long-term debt | $ | 3,014,364 |
|
|
Schedule of GoZone Bonds [Table Text Block] | The following table summarizes the GoZone Bonds outstanding as of December 31, 2016: | | | | | | | | | | | | | | | | | | | | | | | Date Issued | | Maturity Date | | Amount Outstanding | | Amount of Letter of Credit | | Amount Received from Trustee | | Amount Remaining in Trust (a) | | Weighted-Average Interest Rate (b) | | | | | (Thousands of Dollars) | | | June 26, 2008 | | June 1, 2038 | | $ | 55,440 |
| | $ | 56,169 |
| | $ | 55,440 |
| | $ | — |
| | 0.7 | % | July 15, 2010 | | July 1, 2040 | | 100,000 |
| | 101,315 |
| | 100,000 |
| | — |
| | 0.7 | % | October 7, 2010 | | October 1, 2040 | | 50,000 |
| | 50,658 |
| | 43,741 |
| | 6,518 |
| | 0.7 | % | December 29, 2010 | | December 1, 2040 | | 85,000 |
| | 86,118 |
| | 49,782 |
| | 35,841 |
| | 0.7 | % | August 29, 2011 | | August 1, 2041 | | 75,000 |
| | 75,986 |
| | 75,000 |
| | — |
| | 0.7 | % | | | Total | | $ | 365,440 |
| | $ | 370,246 |
| | $ | 323,963 |
| | $ | 42,359 |
| | |
| | (a) | Amount remaining in trust includes accrued interest. |
| | (b) | For the year ended December 31, 2016, our weighted-average interest rate on borrowings was 0.4%. |
|