Summary of Significant Accounting Policies - Additional Information (Detail)			12 Months Ended
	Sep. 01, 2017 USD ($)	Jun. 13, 2016 USD ($)	Dec. 31, 2018 USD ($) payment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2019 USD ($)	Dec. 31, 2012 USD ($)
Accounting Policies [Line Items]
Maturity of federal funds purchased			90 days
Number of consecutive payments for borrower status to be normal, as per consistent repayment record | payment			6
Charges related to assets acquired through foreclosure			$ 300,000	$ 300,000	$ 100,000
Reclassification due to the adoption of ASU No. 2016-01				(1,660,000)
Write-off of unamortized debt issuance costs	$ 700,000		0	695,000	$ 0
Scenario, Plan | Accounting Standards Update 2016-02
Accounting Policies [Line Items]
ROU asset						$ 121,000,000
Lease liability						$ 132,100,000
Senior unsecured fixed - to - floating rate notes
Accounting Policies [Line Items]
Debt instrument, face amount		$ 100,000,000
Interest rate on unsecured debt (as percent)		4.50%
Redemption price, percentage (as percent)		100.00%
Senior unsecured fixed - to - floating rate notes | LIBOR Rate
Accounting Policies [Line Items]
Variable interest rate (as percent)		3.30%
6.25% Senior Notes Due 2019
Accounting Policies [Line Items]
Debt instrument, face amount							$ 55,000,000.0
Interest rate on unsecured debt (as percent)	6.25%						6.25%
Write-off of unamortized debt issuance costs			700,000
Accumulated Other Comprehensive (Loss) Income
Accounting Policies [Line Items]
Reclassification due to the adoption of ASU No. 2016-01			(1,700,000)
Retained Earnings
Accounting Policies [Line Items]
Reclassification due to the adoption of ASU No. 2016-01			$ 1,700,000
Computer Equipment
Accounting Policies [Line Items]
Useful life			3 years
Furniture and equipment
Accounting Policies [Line Items]
Useful life			5 years
Building Renovations
Accounting Policies [Line Items]
Useful life			10 years
Residential
Accounting Policies [Line Items]
Mortgage loans, foreclosures			$ 1,900,000	$ 2,900,000