| (Millions) | ||||||||||
| Marketable securities – auction rate securities only | 2013 | 2012 | 2011 | |||||||
| Beginning balance | $ | 7 | $ | 4 | $ | 7 | ||||
| Total gains or losses: | ||||||||||
| Included in earnings | ― | ― | ― | |||||||
| Included in other comprehensive income | 4 | 3 | (3) | |||||||
| Purchases, issuances, and settlements | ― | ― | ― | |||||||
| Transfers in and/or out of Level 3 | ― | ― | ― | |||||||
| Ending balance (December 31) | 11 | 7 | 4 | |||||||
| Change in unrealized gains or losses for the period included in | ||||||||||
| earnings for securities held at the end of the reporting period | ― | ― | ― | |||||||