Income Taxes - Income Tax Expense from Operations from the Amount Computed by Applying the U.S. Federal Income Tax Statutory Rate of 35% to Income Before Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Income tax computed at federal statutory tax rate	$ 35,338	$ 25,812	$ 20,721
Tax-exempt interest, net	(5,630)	(5,620)	(5,458)
State income tax	762	(146)	504
Dividends received deduction	(19)	(60)	(113)
Other	54	97	(87)
Income tax expense, Total	$ 30,505	$ 20,083	$ 15,567