Basic and Diluted EPS (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Basic EPS:
Net income	$ 66,710	$ 53,747	$ 123,106	$ 102,537
Basic EPS (a)/(b)	$ 0.35	$ 0.28	$ 0.65	$ 0.53
Diluted EPS:
Net income	66,710	53,747	123,106	102,537
Diluted EPS (c)/(d)	$ 0.35	$ 0.28	$ 0.65	$ 0.53
Common Stock
Basic EPS:
Net income	66,710	53,747	123,106	102,537
Less income allocated to nonvested awards	(224)	(191)	(421)	(383)
Net income allocated to common stock for EPS calculation (a)	66,486	53,556	122,685	102,154
Average common shares outstanding (b)	189,070	191,935	188,894	192,390
Basic EPS (a)/(b)	$ 0.35	$ 0.28	$ 0.65	$ 0.53
Diluted EPS:
Net income	66,710	53,747	123,106	102,537
Less income allocated to nonvested awards	(223)	(190)	(419)	(382)
Net income allocated to common stock for EPS calculation (c)	66,487	53,557	122,687	102,155
Average common shares outstanding	189,070	191,935	188,894	192,390
Increase due to assumed issuance of shares related to common equivalent shares outstanding	1,270	459	1,159	366
Average common and common equivalent shares outstanding (d)	190,340	192,394	190,053	192,756
Diluted EPS (c)/(d)	$ 0.35	$ 0.28	$ 0.65	$ 0.53
Participating Securities
Basic EPS:
Less income allocated to nonvested awards	224	191	421	383
Net income allocated to common stock for EPS calculation (a)	224	191	421	383
Average common shares outstanding (b)	650	690	658	727
Basic EPS (a)/(b)	$ 0.35	$ 0.28	$ 0.64	$ 0.53
Diluted EPS:
Less income allocated to nonvested awards	223	190	419	382
Net income allocated to common stock for EPS calculation (c)	$ 223	$ 190	$ 419	$ 382
Average common shares outstanding	650	690	658	727
Average common and common equivalent shares outstanding (d)	650	690	658	727
Diluted EPS (c)/(d)	$ 0.34	$ 0.28	$ 0.64	$ 0.53