Tax Credit Investments - Affordable Housing Tax Credit Investments and Other Tax Credit Investments (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Components of Income Tax Expense
Affordable housing tax credits and other tax benefits		$ 6,757	$ 6,713	$ 7,383
Other tax credit investment credits and tax benefits		10,205	21,351	35,530
Total reduction in income tax expense		16,962	28,064	42,913
Amortization Recorded in Non-Interest Expenses
Amortization of Tax Credit Investments
Affordable housing tax credit investment losses		2,184	1,880	2,748
Affordable housing tax credit investment impairment losses	$ 2,200	3,295	2,544	4,684
Other tax credit investment losses		5,668	1,970	2,866
Other tax credit investment impairment losses	$ 2,100	9,245	17,806	31,449
Total amortization of tax credit investments recorded in non-interest expense		$ 20,392	$ 24,200	$ 41,747