Income Taxes - Components of Income Tax Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Taxes currently payable:
Federal	$ 141	$ 171	$ 148
State	58	76	43
Foreign	13	13	12
Total currently payable	212	260	203
Taxes deferred:
Federal	(34)	69	84
State	(18)	27	(6)
Foreign	(9)	3	(2)
Total deferred	(61)	99	76
Total income tax expense	$ 151	$ 359	$ 279