Earnings Per Share (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Basic earnings per share:
Net income	$ 89,144	$ 77,543	$ 158,448	$ 134,766
Weighted average shares	97,969,074	74,807,174	87,789,596	67,167,275
Basic earnings per share	$ 0.91	$ 1.04	$ 1.80	$ 2.01
Diluted earnings per share:
Net income	$ 89,144	$ 77,543	$ 158,448	$ 134,766
Weighted average shares	97,969,074	74,807,174	87,789,596	67,167,275
Potential dilutive shares from exercise of stock options
Diluted weighted average shares	97,969,074	74,807,174	87,789,596	67,167,275
Diluted earnings per share	$ 0.91	$ 1.04	$ 1.80	$ 2.01