Basic and Diluted EPS under Guidance of ASC 260 (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Basic EPS:
Net income	$ 56,395	$ 48,790
Basic EPS (a)/(b)	$ 0.30	$ 0.25
Diluted EPS:
Net income	56,395	48,790
Diluted EPS (c)/(d)	$ 0.30	$ 0.25
Common Stock
Basic EPS:
Net income	56,395	48,790
Less income allocated to nonvested awards	(196)	(191)
Net income allocated to common stock for EPS calculation (a)	56,199	48,599
Average common shares outstanding (b)	188,052	192,851
Basic EPS (a)/(b)	$ 0.30	$ 0.25
Diluted EPS:
Net income	56,395	48,790
Less income allocated to nonvested awards	(196)	(191)
Net income allocated to common stock for EPS calculation (c)	56,199	48,599
Average common shares outstanding	188,052	192,851
Increase due to assumed issuance of shares related to common equivalent shares outstanding	1,010	305
Average common and common equivalent shares outstanding (d)	189,062	193,156
Diluted EPS (c)/(d)	$ 0.30	$ 0.25
Participating Securities
Basic EPS:
Less income allocated to nonvested awards	196	191
Net income allocated to common stock for EPS calculation (a)	196	191
Average common shares outstanding (b)	667	765
Basic EPS (a)/(b)	$ 0.29	$ 0.25
Diluted EPS:
Less income allocated to nonvested awards	196	191
Net income allocated to common stock for EPS calculation (c)	$ 196	$ 191
Average common shares outstanding	667	765
Average common and common equivalent shares outstanding (d)	667	765
Diluted EPS (c)/(d)	$ 0.29	$ 0.25