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, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income tax computed at federal statutory tax rate		$ 24,258	$ 18,392	$ 28,784
Tax-exempt interest, net		(2,999)	(2,965)	(5,707)
State income tax		1,178	720	311
Dividends received deduction		(59)	(44)	(48)
Revaluation of net deferred income tax assets	$ 12,600			12,620
Other		449	(154)	49
Income tax expense, Total		$ 22,827	$ 15,949	$ 36,009