Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Operating Loss Carryforwards [Line Items]
Effective income tax rate (percent)	30.32%	23.47%	28.31%	24.88%
Decreased deferred tax assets net			$ 13,600,000
Recognized net deferred tax asset	$ 72,720,000		72,720,000		$ 86,352,000
Deferred tax valuation allowance	0		0		0
Tax benefits related to federal NOLs	9,100,000		9,100,000		9,300,000
Unrecognized tax benefit that would impact effective tax rate , if recognized	700,000		700,000		700,000
Penalties recognized associated with unrecognized tax benefits	0	$ 0	0	$ 0
Interest recognized associated with unrecognized tax benefits	0	0	0	0
Accrued amounts for penalties	100,000		100,000		100,000
Accrued amounts for interest	100,000		100,000		100,000
Investment in Low Income Housing Tax Credit	166,100,000		166,100,000		152,700,000
Unfunded Low Income Housing Tax Credit Obligations	63,400,000		63,400,000		61,200,000
Amortization of Low Income Housing Tax Credits	5,500,000	$ 4,800,000	13,700,000	$ 10,400,000
Internal Revenue Service (IRS) [Member]
Operating Loss Carryforwards [Line Items]
Deferred tax asset related to federal and state NOL carryovers	$ 9,100,000		$ 9,100,000		$ 9,300,000