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	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income tax computed at federal statutory tax rate		$ 28,784	$ 39,487	$ 35,338
Tax-exempt interest, net		(5,707)	(5,370)	(5,630)
State income tax		311	913	762
Dividends received deduction		(48)	(125)	(19)
Revaluation of net deferred income tax assets	$ 12,600	12,620
Other		49	51	54
Income tax expense, Total		$ 36,009	$ 34,956	$ 30,505