Accumulated Other Comprehensive Income (Loss) (After-tax Changes by Component) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at beginning of period			$ (26)	$ (23)
Amounts arising during the period			11	44
Reclassifications from accumulated other comprehensive income			(2)	(4)
Total other comprehensive income (loss)	$ 5	$ 36	9	40
Balance at end of period	(17)	17	(17)	17
Available- for-sale securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at beginning of period			194	202
Amounts arising during the period			11	1
Reclassifications from accumulated other comprehensive income			(2)	(2)
Total other comprehensive income (loss)			9	(1)
Balance at end of period	203	201	203	201
Qualifying derivatives
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at beginning of period			44	63
Amounts arising during the period			0	0
Reclassifications from accumulated other comprehensive income			(7)	(10)
Total other comprehensive income (loss)			(7)	(10)
Balance at end of period	37	53	37	53
Prior service costs
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at beginning of period			3	7
Amounts arising during the period			0	(3)
Reclassifications from accumulated other comprehensive income			0	(1)
Total other comprehensive income (loss)			0	(4)
Balance at end of period	3	3	3	3
Actuarial gain (loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at beginning of period			(267)	(295)
Amounts arising during the period			0	46
Reclassifications from accumulated other comprehensive income			7	9
Total other comprehensive income (loss)			7	55
Balance at end of period	$ (260)	$ (240)	$ (260)	$ (240)