Earnings Per Share - Computation of Earnings Per Common Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Numerator:
Net income to common shareholders	$ 46,984	$ 30,378	$ 104,783	$ 57,793
Net income allocated to participating securities - basic and diluted	281	101	844	252
Net income allocated to common shareholders - basic and diluted	$ 46,703	$ 30,277	$ 103,939	$ 57,541
Weighted-average common shares - basic	84,777	84,699	84,757	59,149
Dilutive potential common shares	855	286	768	293
Weighted average common shares - diluted	85,632	84,985	85,525	59,442
Earnings per common share:
Basic	$ 0.55	$ 0.36	$ 1.23	$ 0.97
Diluted	$ 0.55	$ 0.36	$ 1.22	$ 0.97