Quarterly Financial Data (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Data [Abstract]
Revenues	$ 4,428	$ 4,545	$ 4,643	$ 4,084	$ 4,173	$ 4,309	$ 4,616	$ 3,991	$ 17,699	$ 17,088	$ 15,658
Operating income	386	390	433	70	91	51	460	22	1,278	623	693
Income (loss) before income taxes	334	419	363	94	125	33	368	159	1,209	685	323
Net income (loss)	$ 212	$ 259	$ 224	$ 59	$ 78	$ 16	$ 228	$ 98	$ 754	$ 421	$ 178
Basic (in dollars per share)	$ 0.30	$ 0.37	$ 0.31	$ 0.08	$ 0.11	$ 0.02	$ 0.30	$ 0.13	$ 1.06	$ 0.56	$ 0.23
Diluted (in dollars per share)	$ 0.30	$ 0.37	$ 0.31	$ 0.08	$ 0.11	$ 0.02	$ 0.30	$ 0.13	$ 1.05	$ 0.56	$ 0.23