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, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Numerator:
Net income to common shareholders	$ 33,202	$ 46,984	$ 128,640	$ 104,783
Net income allocated to participating securities - basic and diluted	616	281	2,398	844
Net income allocated to common shareholders - basic and diluted	$ 32,586	$ 46,703	$ 126,242	$ 103,939
Denominator:
Weighted-average common shares - basic	82,091	84,777	83,404	84,757
Dilutive potential common shares	114	855	92	768
Weighted average common shares - diluted	82,205	85,632	83,496	85,525
Earnings per common share:
Basic	$ 0.40	$ 0.55	$ 1.51	$ 1.23
Diluted	$ 0.40	$ 0.55	$ 1.51	$ 1.22