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, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of Income Tax Expense
Affordable housing tax credits and other tax benefits		$ 7,383	$ 5,013	$ 4,709
Other tax credit investment credits and tax benefits		35,530	33,294	23,877
Total reduction in income tax expense		42,913	38,307	28,586
Amortization Recorded in Non-Interest Expenses
Amortization of Tax Credit Investments
Affordable housing tax credit investment losses		2,748	2,077	2,594
Affordable housing tax credit investment impairment losses	$ 2,200	4,684	450	1,321
Other tax credit investment losses		2,866	790	1,079
Other tax credit investment impairment losses	$ 2,100	31,449	31,427	22,318
Total amortization of tax credit investments recorded in non-interest expense		$ 41,747	$ 34,744	$ 27,312