Unaudited Quarterly Financial Data - Summary of Financial Data (Detail) (USD $) In Thousands, 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
Schedule Of Quarterly Financial Data [Line Items]
Revenues	$ 750,537	$ 729,141	$ 738,188	$ 768,642	$ 748,357	$ 878,177	$ 889,496	$ 806,389	$ 2,986,508	$ 3,322,419	$ 3,322,974
Operating income	194,962	244,822	257,184	265,410	218,276	350,277	367,596	319,265	962,378	1,255,414	1,425,374
Income before income tax expense	185,041	234,847	246,758	251,435	203,547	334,849	344,026	296,849	918,081	1,179,271	1,336,016
Net income	$ 155,661	$ 178,186	$ 201,461	$ 185,169	$ 188,490	$ 256,854	$ 266,586	$ 250,612	$ 720,477	$ 962,542	$ 955,457
Net income per share:
Basic	$ 1.12	$ 1.28	$ 1.45	$ 1.33	$ 1.36	$ 1.85	$ 1.92	$ 1.80	$ 5.18	$ 6.92	$ 6.87
Diluted	$ 1.12	$ 1.28	$ 1.45	$ 1.33	$ 1.36	$ 1.85	$ 1.92	$ 1.80	$ 5.18	$ 6.92	$ 6.87