Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Basic:
Net income (loss)	$ 215,160	$ 223,784	$ (11,351)	$ (58,177)	$ 149,931	$ 331,836	$ 28,325	$ (11,218)	$ 369,416	$ 498,874	$ 903,944
Less: Net income allocable to participating RSUs									(3,179)	(4,028)	(8,335)
Net income available for basic common shares									$ 366,237	$ 494,846	$ 895,609
Weighted average number of common shares (in shares)	339,815	339,420	338,843	338,579	338,416	338,728	338,709	340,226	339,172	339,016	343,394
Net Income Per Common Share-Basic (in USD per share)	$ 0.63	$ 0.66	$ (0.03)	$ (0.17)	$ 0.44	$ 0.97	$ 0.08	$ (0.03)	$ 1.08	$ 1.46	$ 2.61
Diluted:
Net income (loss)	$ 215,160	$ 223,784	$ (11,351)	$ (58,177)	$ 149,931	$ 331,836	$ 28,325	$ (11,218)	$ 369,416	$ 498,874	$ 903,944
Less: Net income allocable to participating RSUs									(3,179)	(4,028)	(8,291)
Net income available for diluted common shares									$ 366,237	$ 494,846	$ 895,653
Weighted average number of common shares (in shares)	339,815	339,420	338,843	338,579	338,416	338,728	338,709	340,226	339,172	339,016	343,394
Weighted average common equivalent shares arising from:
Dilutive stock options and non-participating RSUs (in shares)									576	1,752	4,065
Weighted average number of common and potential common shares (in shares)	340,364	339,790	338,843	338,579	339,506	340,329	340,644	340,226	339,748	340,768	347,459
Net Income Per Common Share-Diluted (in USD per share)	$ 0.63	$ 0.66	$ (0.03)	$ (0.17)	$ 0.44	$ 0.97	$ 0.08	$ (0.03)	$ 1.08	$ 1.45	$ 2.58