Income Taxes - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal corporate income tax rate, percent		35.00%		35.00%		35.00%
Increase to income tax rate provision as a result of Federal corporate tax rate change	$ 865	$ 865	[1]	$ 0	[1]	$ 0	[1]
Unrecognized tax benefit, affect the effective tax rate	13	13		10		14
Accruals for payment of interest and penalties associated with uncertain tax positions	4	4		3		$ 4
Increase (decrease) in accruals for payment of interest and penalties associated with uncertain tax positions	1	1		(1)
Net deferred tax asset	$ 1,540	$ 1,540		$ 1,803

[1] For 2017, the amount relates to the effects of revaluing our net deferred tax asset for new Tax Legislation that was enacted on December 22, 2017.