Unaudited Quarterly Financial Data (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 2,360	$ 2,287	$ 2,259	$ 2,202	$ 2,148	$ 2,118	$ 1,972	$ 1,917	$ 9,108	$ 8,155	$ 7,504
Income from operations	277	218	236	240	244	209	234	222	971	909	915
Net income (loss)	$ (48)	$ (53)	$ (45)	$ (37)	$ 39	$ (70)	$ (96)	$ (42)	$ (183)	$ (169)	$ (304)
Loss per common share, basic and diluted (in dollars per share)	$ (0.44)	$ (0.49)	$ (0.42)	$ (0.35)		$ (0.68)	$ (0.96)	$ (0.42)	$ (1.70)	$ (1.65)	$ (3.05)
Income per common share, basic (in dollars per share)					$ 0.38
Income per common share, diluted (in dollars per share)					$ 0.35
Weighted average common shares outstanding, basic and diluted (in shares)	110,242,507	108,792,605	107,975,937	106,439,198		102,924,443	100,600,678	100,327,418	108,374,160	101,934,630	99,657,989
Weighted average common shares outstanding, basic (in shares)					103,836,535
Weighted average common shares outstanding, diluted (in shares)					111,415,982