Income Taxes - Components of Income 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:
Federal									$ 50,980	$ 40,949	$ 36,367
State									6,216	5,540	7,462
Current Income Tax Expense (Benefit), Total									57,196	46,489	43,829
Deferred:
Federal									2,734	4,209	(448)
State									418	898	32
Deferred Income Tax Expense (Benefit)									3,152	5,107	(416)
Tax expense recorded as an increase of paid-in capital									7,884	4,588	564
Income Tax Expense (Benefit), Total	$ 20,404	$ 16,407	$ 14,659	$ 16,762	$ 14,215	$ 15,714	$ 12,048	$ 14,207	$ 68,232	$ 56,184	$ 43,977