Qualified Affordable Housing Projects and Other Tax Credit Investments - Schedule of Qualified Affordable Housing Projects and Other Tax Credit Investments (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Investment Holdings [Line Items]
Total	$ 50,299		$ 50,299		$ 64,896
Total	41,040		41,040		48,866
Amortization Expense	9,874	$ 940	23,730	$ 2,822
Tax Expense (Benefit) Recognized	(5,199)	(3,120)	(17,054)	(9,360)
LIHTC [Member]
Investment Holdings [Line Items]
Proportional amortization method of investment	28,991		28,991		31,183
Unfunded commitment, Proportional amortization	5,134		5,134		15,553
Amortization Expense	640	940	1,923	2,822
Tax Expense (Benefit) Recognized	(832)	(1,297)	(2,494)	(3,892)
FHTC [Member]
Investment Holdings [Line Items]
Equity method investment	2,039		2,039		10,645
Unfunded commitment, Equity	7,760		7,760		12,040
Amortization Expense	3,282		8,727
Tax Expense (Benefit) Recognized	(1,258)	(1,520)	(5,154)	(4,559)
CReED [Member]
Investment Holdings [Line Items]
Equity method investment	17		17		704
Unfunded commitment, Equity	538		538		1,502
Amortization Expense	687		687
Tax Expense (Benefit) Recognized		$ (303)		$ (909)
Renewable Energy [Member]
Investment Holdings [Line Items]
Equity method investment	19,252		19,252		22,364
Unfunded commitment, Equity	27,608		27,608		$ 19,771
Amortization Expense	5,265		12,393
Tax Expense (Benefit) Recognized	$ (3,109)		$ (9,406)