Calculation of Basic and Diluted Net Income per Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
BASIC EPS:
Net income	$ 15,976	$ 74,420	$ 105,081	$ 171,418
Adjust for income attributed to holders of non-vested restricted stock	(160)	(808)	(1,074)	(1,660)
Income attributed to common stockholders	15,816	73,612	104,007	169,758
Weighted average number of common shares outstanding, excluding non-vested shares of restricted stock	144,798	144,446	143,778	144,915
Basic net income per common share	$ 0.11	$ 0.51	$ 0.72	$ 1.17
DILUTED EPS:
Income attributed to common stockholders	$ 15,816	$ 73,612	$ 104,007	$ 169,758
Weighted average number of common shares outstanding, excluding non-vested shares of restricted stock	144,798	144,446	143,778	144,915
Add dilutive effect of potential common shares	2,193	986	2,323	925
Weighted average number of diluted common shares outstanding	146,991	145,432	146,101	145,840
Diluted net income per common share	$ 0.11	$ 0.51	$ 0.71	$ 1.16
Potentially dilutive securities excluded as anti-dilutive	442	2,897	473	4,043