Calculation of Net Income Per Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Net income	$ 57,660	$ 44,168	$ 151,323	$ 107,298
Net income available to common stockholders-basic	57,382	43,553	150,547	105,734
Basic weighted average shares	137,918	139,449	138,627	141,118
Basic net income per share	$ 0.42	$ 0.31	$ 1.09	$ 0.75
Net income	57,660	44,168	151,323	107,298
Net income available to common stockholders-diluted	57,383	43,556	150,551	105,740
Basic weighted average shares	137,918	139,449	138,627	141,118
Dilutive effect of potential common shares	1,223	984	1,108	1,255
Diluted weighted average shares	139,141	140,433	139,735	142,373
Diluted net income per share	$ 0.41	$ 0.31	$ 1.08	$ 0.74
Basic
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income allocated to participating securities	278	615	776	1,564
Diluted
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income allocated to participating securities	$ 277	$ 612	$ 772	$ 1,558