Tax Credit Investments Tax Credit Investments - Income Statement Disclosures (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Components of Income Tax Expense:
Affordable housing tax credits and other tax benefits	$ 1,284	$ 1,065
Other tax credit investment credits and tax benefits	6,286	3,268
Total reduction in income tax expense	7,570	4,333
Non-Interest Expenses
Amortization of Tax Credit Investments:
Affordable housing tax credit investment losses	396	584
Affordable housing tax credit investment impairment losses	124	140
Other tax credit investment losses	767	74
Other tax credit investment impairment losses	4,037	6,466
Total amortization of tax credit investments recorded in non-interest expense	$ 5,324	$ 7,264