Calculation of Net Income Per Share (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
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Net income	$ 45,329	$ 36,425	$ 93,663	$ 63,130
Net income available to common stockholders-basic	45,101	35,888	93,169	62,191
Basic weighted average shares	138,588	141,286	138,986	141,966
Basic net income per share	$ 0.33	$ 0.25	$ 0.67	$ 0.44
Net income	45,329	36,425	93,663	63,130
Net income available to common stockholders-diluted	45,101	35,890	93,171	62,194
Basic weighted average shares	138,588	141,286	138,986	141,966
Dilutive effect of potential common shares	1,151	1,299	1,050	1,391
Diluted weighted average shares	139,739	142,585	140,036	143,357
Diluted net income per share	$ 0.32	$ 0.25	$ 0.67	$ 0.43
Basic
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income allocated to participating securities	228	537	494	939
Diluted
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income allocated to participating securities	$ 228	$ 535	$ 492	$ 936