Fair Value Measurement of Assets and Liabilities - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Fair Value Assets Measured On Recurring Basis
Impaired collateral dependent loans, charge-offs			$ 0	$ 2,100,000
Collateral dependent impaired loans, recorded investment	$ 60,200,000		60,200,000
Specific valuation allowance allocations	23,200,000		23,200,000
Reported net carrying amount of impaired loans	37,000,000		$ 37,000,000
Valuation of loan servicing rights, discount rate			8.00%
Net recoveries of net impairment charges	48,000	$ 134,000	$ 365,000	185,000
Foreclosed assets measured at fair value upon initial recognition			4,500,000
Foreclosed asset charge-offs	267,000	536,000	1,500,000	1,500,000
Loss due to re-measurement of repossessed assets	$ 245,000	$ 0	$ 390,000	$ 290,000
Minimum
Fair Value Assets Measured On Recurring Basis
Valuation of loan servicing rights, prepayment rate			0.00%
Maximum
Fair Value Assets Measured On Recurring Basis
Valuation of loan servicing rights, prepayment rate			24.00%