Tax Credit Investments 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, 2018	Dec. 31, 2017	Dec. 31, 2016
Components of Income Tax Expense
Affordable housing tax credits and other tax benefits		$ 6,713	$ 7,383	$ 5,013
Other tax credit investment credits and tax benefits		21,351	35,530	33,294
Total reduction in income tax expense		28,064	42,913	38,307
Amortization Recorded in Non-Interest Expenses
Amortization of Tax Credit Investments
Affordable housing tax credit investment losses		1,880	2,748	2,077
Affordable housing tax credit investment impairment losses	$ 2,200	2,544	4,684	450
Other tax credit investment losses		1,970	2,866	790
Other tax credit investment impairment losses	$ 2,100	17,806	31,449	31,427
Total amortization of tax credit investments recorded in non-interest expense		$ 24,200	$ 41,747	$ 34,744