Earnings Per Share - Computation of Earnings Per Common Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Numerator:
Net income to common shareholders	$ 151,742	$ 76,759	$ 52,206
Net income allocated to participating securities - basic and diluted	1,557	866	320
Net income allocated to common shareholders - basic and diluted	$ 150,185	$ 75,893	$ 51,886
Denominator:
Weighted-average common shares - basic	84,767	65,590	36,876
Dilutive potential common shares	821	480	178
Weighted average common shares - diluted	85,588	66,070	37,054
Earnings per common share:
Basic	$ 1.77	$ 1.16	$ 1.41
Diluted	$ 1.75	$ 1.15	$ 1.40