Quarterly Financial Data (Unaudited) (Summary Of Quarterly Financial Data) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Data [Abstract]
Revenues	$ 3,462	$ 3,453	$ 3,370	$ 3,202	$ 3,233	$ 3,187	$ 3,085	$ 2,911	$ 13,487	$ 12,416	$ 10,263
Cost of revenues (exclusive of depreciation and amortization expense shown separately below)	2,078	2,077	2,038	1,915	1,933	1,935	1,845	1,727	8,108	7,440	6,141
Selling, general and administrative expenses	730	701	654	646	659	627	612	611	2,731	2,509	2,037
Depreciation and amortization expense	93	92	87	87	88	82	82	73	359	325	200
Income from operations	561	583	591	554	553	543	546	500	2,289	2,142	1,885
Net income	$ 416	$ 444	$ 252	$ 441	$ 424	$ 397	$ 420	$ 383	$ 1,553	$ 1,624	$ 1,439
Basic earnings per share	$ 0.69	$ 0.73	$ 0.42	$ 0.73	$ 0.70	$ 0.65	$ 0.69	$ 0.63	$ 2.56	$ 2.67	$ 2.37
Diluted earnings per share	$ 0.68	$ 0.73	$ 0.41	$ 0.72	$ 0.69	$ 0.65	$ 0.68	$ 0.62	$ 2.55	$ 2.65	$ 2.35