Calculation of Basic and Diluted 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
BASIC EPS:
Net income	$ 40,768	$ 92,538	$ 96,998	$ 189,812
Adjust for income attributed to holders of non-vested restricted stock	(392)	(806)	(882)	(1,550)
Income attributed to common stockholders	40,376	91,732	96,116	188,262
Weighted average number of common shares outstanding, excluding non-vested shares of restricted stock	145,465	153,269	145,148	153,947
Basic net income per common share	$ 0.28	$ 0.60	$ 0.66	$ 1.22
DILUTED EPS:
Income attributed to common stockholders	40,376	91,732	96,116	188,262
Add incremental earnings related to potential common shares	2		6
Adjusted income attributed to common stockholders	$ 40,378	$ 91,732	$ 96,122	$ 188,262
Weighted average number of common shares outstanding, excluding non-vested shares of restricted stock	145,465	153,269	145,148	153,947
Add dilutive effect of potential common shares	909	386	1,144	597
Weighted average number of diluted common shares outstanding	146,374	153,655	146,292	154,544
Diluted net income per common share	$ 0.28	$ 0.60	$ 0.66	$ 1.22
Potentially dilutive securities excluded as anti-dilutive	4,015	5,969	4,065	5,529