Common Stock (Narrative) (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($)
Class of Stock [Line Items]
Common stock authorized (in shares)	500,000,000	500,000,000
Common stock, par value (in dollars per share) | $ / shares	$ 0.01	$ 0.01
Employee stock option
Class of Stock [Line Items]
Common stock, capital shares reserved for future issuance (in shares)	500,000	500,000
Restricted Stock Units (RSUs)
Class of Stock [Line Items]
Restricted stock units outstanding (in shares)	1,000,000
Share conversion ratio	1
Shares issued for RSUs (in shares)	346,000
Shares paid for tax withholding (in shares)	227,000
Compensation expense not yet recognized | $	$ 46
Compensation expense not yet recognized, period for recognition	2 years 1 month
Fair value of RSUs vested during the period | $	$ 101	$ 39	$ 84
Time-based Restricted Stock Units
Class of Stock [Line Items]
Vesting period	3 years
Vesting period, start duration from grant date	12 months
2010 Long Term Incentive Plan
Class of Stock [Line Items]
Shares available for grant (in shares)	2,600,000