Income Taxes - Income Tax Expense from Operations from the Amount Computed by Applying the U.S. Federal Income Tax Statutory Rate of 21% to Income Before Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income tax computed at federal statutory tax rate	$ 22,453	$ 24,258	$ 18,392
Tax-exempt interest, net	(3,226)	(2,999)	(2,965)
State income tax	1,080	1,178	720
Dividends received deduction	(89)	(59)	(44)
Other	99	449	(154)
Income tax expense, Total	$ 20,317	$ 22,827	$ 15,949