Tax Credit Investments - Income Statement Disclosures (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Components of Income Tax Expense:
Affordable housing tax credits and other tax benefits	$ 1,271	$ 1,065	$ 2,555	$ 2,130
Other tax credit investment credits and tax benefits	8,680	3,268	14,966	6,536
Total reduction in income tax expense	9,951	4,333	17,521	8,666
Non-Interest Expenses
Amortization of Tax Credit Investments:
Affordable housing tax credit investment losses	358	775	754	1,359
Affordable housing tax credit investment impairment losses	130	60	254	200
Other tax credit investment losses	1,060	594	1,827	668
Other tax credit investment impairment losses	6,184	6,217	10,221	12,683
Total amortization of tax credit investments recorded in non-interest expense	$ 7,732	$ 7,646	$ 13,056	$ 14,910