Quarterly Financial Information (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Financial Information (Unaudited)
Revenue	$ 1,226	$ 1,116	$ 1,072	$ 1,035	$ 1,004	$ 971	$ 1,020	$ 959	$ 4,450	$ 3,954	$ 3,639
Income from continuing operations	211	172	180	163	113	167	181	165	726	626	599
(Loss) Income from discontinued operations	(404)	165	40	(36)	108	207	34	(41)	(234)	308	252
Net income (loss)	(193)	337	220	127	221	374	216	124	492	934	851
Income from continuing operations	190	151	176	158	108	161	177	160	676	607	581
(Loss) income from discontinued operations	(406)	162	39	(35)	106	205	34	(40)	(239)	304	247
Net income (loss)	$ (216)	$ 314	$ 216	$ 123	$ 214	$ 366	$ 211	$ 120	$ 437	$ 911	$ 828
Basic, Income from continuing operations (in usd per share)	$ 0.68	$ 0.54	$ 0.63	$ 0.57	$ 0.38	$ 0.54	$ 0.59	$ 0.52	$ 2.43	$ 2.03	$ 1.88
Basic, Income from discontinued operations (in usd per share)	$ (1.46)	$ 0.58	$ 0.14	$ (0.13)	$ 0.37	$ 0.69	$ 0.11	$ (0.13)	$ (0.86)	$ 1.02	$ 0.80
Basic, Net income (in usd per share)	$ (0.78)	$ 1.13	$ 0.77	$ 0.44	$ 0.75	$ 1.23	$ 0.70	$ 0.39	$ 1.57	$ 3.05	$ 2.68
Diluted, Income from continuing operations (in usd per share)	$ 0.67	$ 0.53	$ 0.62	$ 0.56	$ 0.37	$ 0.53	$ 0.57	$ 0.52	$ 2.37	$ 2.00	$ 1.86
Diluted, Income (loss) from discontinued operations, net of tax (in usd per share)	$ (1.43)	$ 0.57	$ 0.14	$ (0.13)	$ 0.36	$ 0.67	$ 0.11	$ (0.13)	$ (0.84)	$ 1.00	$ 0.79
Diluted, Net income (in usd per share)	$ (0.76)	$ 1.10	$ 0.76	$ 0.43	$ 0.73	$ 1.21	$ 0.68	$ 0.39	$ 1.53	$ 3.00	$ 2.65