Quarterly Financial Information (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
Disclosure Unaudited Quarterly Financial Information [Abstract]
Revenues	$ 1,172	$ 1,152	$ 1,104	$ 1,103	$ 1,070	$ 1,035	$ 1,020	$ 986	$ 4,531	$ 4,111	$ 3,803
Income (loss) before income taxes	167	182	148	217	273	152	29	112	714	566	173
Net income (loss)	$ 122	$ 131	$ 110	$ 154	$ 192	$ 111	$ 32	$ 86	$ 517	$ 421	$ 164
Net income (loss) per common share-basic	$ 0.75	$ 0.80	$ 0.67	$ 0.95	$ 1.18	$ 0.69	$ 0.20	$ 0.53	$ 3.16	$ 2.59	$ 1.01
Net income (loss) per common share-diluted	$ 0.74	$ 0.79	$ 0.66	$ 0.94	$ 1.17	$ 0.68	$ 0.20	$ 0.53	$ 3.12	$ 2.57	$ 1.01