Qualified Affordable Housing Projects and Other Tax Credit Investments - Schedule of Qualified Affordable Housing Projects and Other Tax Credit Investments (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment Holdings [Line Items]
Total	$ 64,896,000	$ 35,048,000
Total	48,866,000	20,816,000
Amortization Expense	13,655	804	$ 947
Tax Benefit Recognized	(15,701)	(1,125)	(1,358)
LIHTC [Member]
Investment Holdings [Line Items]
Proportional amortization method of investment	31,183,000	29,110,000
Unfunded commitment, Proportional amortization	15,553,000	16,210,000
Amortization Expense	1,922	804	804
Tax Benefit Recognized	(2,666)	(1,125)	(1,125)
FHTC [Member]
Investment Holdings [Line Items]
Equity method investment	10,645,000	4,434,000
Unfunded commitment, Equity	12,040,000	3,104,000
Amortization Expense	10,441
Tax Benefit Recognized	(11,348)
CReED [Member]
Investment Holdings [Line Items]
Equity method investment	704,000	1,504,000
Unfunded commitment, Equity	1,502,000	$ 1,502,000
Amortization Expense	800
Tax Benefit Recognized	(1,074)
Renewable Energy [Member]
Investment Holdings [Line Items]
Equity method investment	22,364,000
Unfunded commitment, Equity	19,771,000
Amortization Expense	492
Tax Benefit Recognized	$ (613)
NMTC [Member]
Investment Holdings [Line Items]
Amortization Expense			143
Tax Benefit Recognized			$ (233)