Income Taxes (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Income Tax Contingency [Abstract]
Unrecognized benefit that would affect the effective tax rate	$ 7
Amount of interest and penalties included in liabilities for uncertain tax positions	$ 4
Effective Income Tax Rate, Continuing Operations	53.80%	0.10%
U.S. federal corporate tax rate	21.00%		35.00%
Valuation allowance as a result of an enacted tax law change	$ 15
Tax expense related to tax deficiency from stock-based compensation	4
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Provisional Income Tax Expense (Benefit)			$ 32
Cash Paid for Income Taxes, Net of Refunds Received [Abstract]
Cash paid for income taxes (refunds received), net
Maximum [Member]
Cash Paid for Income Taxes, Net of Refunds Received [Abstract]
Cash paid for income taxes (refunds received), net	$ (1)	$ 1