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, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Numerator:
Net income to common shareholders	$ 46,553	$ 33,202	$ 135,630	$ 128,640
Net income allocated to participating securities - basic and diluted	883	616	2,783	2,398
Net income allocated to common shareholders - basic and diluted	$ 45,670	$ 32,586	$ 132,847	$ 126,242
Denominator:
Weighted average common shares - basic	81,721	82,091	81,965	83,404
Dilutive potential common shares	221	114	239	92
Weighted average common shares - diluted	81,942	82,205	82,204	83,496
Earnings per common share:
Basic	$ 0.56	$ 0.40	$ 1.62	$ 1.51
Diluted	$ 0.56	$ 0.40	$ 1.62	$ 1.51