Income Taxes - Impact of U.S. Tax Reform (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Impact of U.S. Tax Reform [Line Items]
U.S. statutory tax rate			21.00%	35.00%	35.00%
Tax Cuts and Jobs Act, Incomplete Accounting, Change in Federal Corporate Tax Rate, Provisional Income Tax Expense (Benefit)					$ 208
Tax Cuts and Jobs Act, Incomplete Accounting, Repatriation of Foreign Earnings, Provisional Income Tax Expense (Benefit)					1
Tax Cuts and Jobs Act, Incomplete Accounting, Bonus Depreciation, Provisional Income Tax Expense (Benefit)					40
Tax Cuts and Jobs Act, Incomplete Accounting, Executive Compensation, Provisional Income Tax Expense (Benefit)					8
Benefit (provision) for income taxes	$ (10)	$ 19	$ 42	$ 73
Effective tax rate	(28.10%)	(53.00%)	11.30%	17.70%
Liabilities for uncertain tax positions	$ 51		$ 51
State and Local Jurisdiction [Member]
Impact of U.S. Tax Reform [Line Items]
Tax Cuts and Jobs Act, Incomplete Accounting, One Time Transition Tax Liability For Foreign Subsidiaries, Provisional Income Tax Expense					2
Internal Revenue Service (IRS) [Member]
Impact of U.S. Tax Reform [Line Items]
Tax Cuts and Jobs Act, Incomplete Accounting, One Time Transition Tax Liability For Foreign Subsidiaries, Provisional Income Tax Expense					$ 70
Tax Cuts and Jobs Act, Incomplete Accounting, Change In Tax Rate One Time Transition Tax Liability, Provisional Income Tax Benefit			64
Minimum [Member]
Impact of U.S. Tax Reform [Line Items]
Expected decrease in liability for uncertain tax position	2		$ 2
Maximum [Member]
Impact of U.S. Tax Reform [Line Items]
GILTI deduction rate			50.00%
Expected decrease in liability for uncertain tax position	$ 4		$ 4