Summary of Significant Accounting Policies - Reconciliation of Numerators and Denominators of Basic and Diluted Earnings Per Share Calculations (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Accounting Policies [Abstract]
Net income	$ 16,351	$ 19,228	$ 33,380	$ 37,056
Income available to common stockholders, Basic	16,351	19,228	33,380	37,056
Income available to common stockholders, Diluted	$ 16,351	$ 19,228	$ 33,380	$ 37,056
Weighted Average Common Shares Outstanding, Basic	59,147,241	57,206,242	58,263,052	57,186,828
Dilutive shares	122,021	77,404	140,701	54,104
Weighted Average Common Shares Outstanding, Diluted	59,269,262	57,283,646	58,403,753	57,240,932
Income available to common stockholders, Per Share Amount, Basic	$ 0.28	$ 0.34	$ 0.57	$ 0.65
Income available to common stockholders, Per Share Amount, Diluted	$ 0.28	$ 0.34	$ 0.57	$ 0.65