Earnings Per Share - Computation of Earnings Per Common Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Numerator:
Net income to common shareholders	$ 39,962	$ 46,862	$ 89,077	$ 95,438
Net income allocated to participating securities - basic and diluted	819	880	1,900	1,782
Net income allocated to common shareholders - basic and diluted	$ 39,143	$ 45,982	$ 87,177	$ 93,656
Denominator:
Weighted-average common shares - basic	81,933	83,279	82,099	84,071
Dilutive potential common shares	241	78	249	82
Weighted average common shares - diluted	82,174	83,357	82,348	84,153
Earnings per common share:
Basic	$ 0.48	$ 0.55	$ 1.06	$ 1.11
Diluted	$ 0.48	$ 0.55	$ 1.06	$ 1.11