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, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income tax computed at federal statutory tax rate		$ 18,392	$ 28,784	$ 39,487
Tax-exempt interest, net		(2,965)	(5,707)	(5,370)
State income tax		720	311	913
Dividends received deduction		(44)	(48)	(125)
Revaluation of net deferred income tax assets	$ 12,600		12,620
Other		(154)	49	51
Income tax expense, Total		$ 15,949	$ 36,009	$ 34,956