Schedule of Net Income Attributable to Common Stockholders (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 115,501	$ 81,049	$ 203,314	$ 164,593
Less: Net income attributable to participating securities	(606)	(715)	(1,240)	(1,492)
Net income attributable to common shareholders	$ 114,895	$ 80,334	$ 202,074	$ 163,101