Debt - Extinguishment (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended				12 Months Ended
	Apr. 30, 2015	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Apr. 13, 2015
Extinguishment of Debt [Line Items]
Extinguishment of Debt, Amount			$ 1,000
Debt redeemed, including accrued interest and expenses									$ 955
Debt extinguishment costs	$ 100		321			$ 321	$ 0	$ 100
Debt extinguishment costs, after-tax	$ 65	$ 0	$ 209	$ 0	$ 0	$ 209		$ 65