Tax Credit Investments - Income Statement Disclosures (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Components of Income Tax Expense
Affordable housing tax credits and other tax benefits	$ 1,965	$ 1,065	$ 4,520	$ 3,195
Other tax credit investment credits and tax benefits	7,859	6,118	22,825	12,654
Total reduction in income tax expense	9,824	7,183	27,345	15,849
Non-Interest Expenses
Amortization of Tax Credit Investments
Affordable housing tax credit investment losses	1,183	33	1,937	1,392
Affordable housing tax credit investment impairment losses	979	128	1,233	328
Other tax credit investment losses	307	107	2,134	775
Other tax credit investment impairment losses	5,920	6,182	16,141	18,865
Total amortization of tax credit investments recorded in non-interest expense	$ 8,389	$ 6,450	$ 21,445	$ 21,360