Income Taxes - Current and Deferred Tax Expense (Benefit) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current:
Current Federal									$ 27,577	$ 27,667	$ 29,813
Current State									542	2,168	176
Total current									28,119	29,835	29,989
Deferred:
Deferred Federal									1,678	4,210	(3,208)
Deferred State									1,988	1,321	2,074
Total deferred									3,666	5,531	(1,134)
Income tax expense (benefit)	$ 9,968	$ 7,913	$ 6,206	$ 7,698	$ 7,806	$ 11,987	$ 8,007	$ 7,566	$ 31,785	$ 35,366	$ 28,855