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, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Net income	$ 63,089	$ 45,329	$ 118,952	$ 93,663
Income allocated to participating securities-basic	1	228	4	494
Net income available to common stockholders-basic	63,088	45,101	118,948	93,169
Basic weighted average shares	136,385	138,588	136,845	138,986
Basic net income per share	$ 0.46	$ 0.33	$ 0.87	$ 0.67
Net income	63,089	45,329	118,952	93,663
Income allocated to participating securities-diluted	1	228	4	492
Net income available to common stockholders-diluted	$ 63,088	$ 45,101	$ 118,948	$ 93,171
Basic weighted average shares	136,385	138,588	136,845	138,986
Dilutive effect of potential common shares	1,274	1,151	1,179	1,050
Diluted weighted average shares	137,659	139,739	138,024	140,036
Diluted net income per share	$ 0.46	$ 0.32	$ 0.86	$ 0.67