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, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Income tax computed at federal statutory tax rate	$ 39,487	$ 35,338	$ 25,812
Tax-exempt interest, net	(5,370)	(5,630)	(5,620)
State income tax	913	762	(146)
Dividends received deduction	(125)	(19)	(60)
Other	51	54	97
Income tax expense, Total	$ 34,956	$ 30,505	$ 20,083