Quarterly Financial Data (Unaudited) (Details) (USD $) In Millions, except Per 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 Data [Abstract]
Revenues	$ 2,378		$ 2,411		$ 2,440		$ 2,463		$ 2,569		$ 2,458		$ 2,450		$ 2,455		$ 9,692		$ 9,932		$ 9,364
Net income (loss)	$ 198	[1]	$ 213	[1]	$ 267	[1]	$ 13	[1]	$ 221		$ 272		$ 194		$ 250		$ 691	[1]	$ 937		$ 628
Basic earnings (loss) per share	$ 0.73		$ 0.79		$ 0.99		$ 0.05		$ 0.82		$ 1.01		$ 0.72		$ 0.93		$ 2.56		$ 3.48		$ 2.33
Diluted earnings (loss) per share	$ 0.73		$ 0.79		$ 0.98		$ 0.05		$ 0.82	[2]	$ 1.01	[2]	$ 0.72	[2]	$ 0.93	[2]	$ 2.55		$ 3.47	[2]	$ 2.33

[1] Net income in the first quarter of 2014 includes the impairment loss on the sale of CAC.

[2] Due to the averaging of shares, quarterly earnings per share do not add to the total for the full year.