Reconciliation between Basic and Diluted Earnings Per Share (Detail) (USD $) In Thousands, except Per Share data	3 Months Ended							6 Months Ended			9 Months Ended		12 Months Ended
	Jun. 30, 2011	Mar. 31, 2011	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Sep. 30, 2009	Jun. 30, 2009	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2009	Sep. 30, 2010	Sep. 30, 2009	Dec. 31, 2010	Dec. 31, 2009
Earnings Per Share Disclosure [Line Items]
Net income for basic and diluted earnings per share	$ 93,491	$ 78,410	$ 350,179	$ 53,731	$ 79,843	$ 229,708	$ (36,276)	$ 171,901	$ 133,574	$ (72,514)	$ 483,753	$ 157,195	$ 515,653	$ 152,485
Weighted-average common shares outstanding	217,028			215,043				216,771	214,680
Dilutive effect of stock options and restricted stock awards	2,342			2,518				2,745	2,624
Diluted weighted-average shares outstanding	219,370			217,561				219,516	217,304
Basic earnings per share	$ 0.43	$ 0.36	$ 1.63	$ 0.25	$ 0.37	$ 1.07	$ (0.17)	$ 0.79	$ 0.62	$ (0.34)	$ 2.25	$ 0.74	$ 2.40	$ 0.71
Diluted earnings per share	$ 0.43	$ 0.36	$ 1.61	$ 0.25	$ 0.37	$ 1.07	$ (0.17)	$ 0.78	$ 0.61	$ (0.34)	$ 2.23	$ 0.73	$ 2.37	$ 0.71