Calculation of Net Income Per Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Net income	$ 55,863	$ 48,334
Net income available to common stockholders-basic	55,861	48,069
Basic weighted average shares	137,311	139,384
Basic net income per share	$ 0.41	$ 0.34
Net income	55,863	48,334
Net income available to common stockholders-diluted	55,861	48,070
Basic weighted average shares	137,311	139,384
Dilutive effect of potential common shares	1,083	1,059
Diluted weighted average shares	138,394	140,443
Diluted net income per share	$ 0.40	$ 0.34
Basic
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income allocated to participating securities	2	265
Diluted
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income allocated to participating securities	$ 2	$ 264