Leases - Narrative (Details) - USD ($)	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Operating Lease, Right-of-Use Asset	$ 18,011,000			$ 13,714,000
Operating Lease, Liability	$ 19,127,000			$ 14,482,000
Operating Lease, Weighted Average Remaining Lease Term	7 years 8 months 12 days			7 years 2 months 12 days
Operating Lease, Weighted Average Discount Rate, Percent	2.40%			3.10%
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 1,016,996,000			$ 969,014,000
Deferred Rent Credit								$ 600,000
Other Assets [Member]
Operating Lease, Right-of-Use Asset	18,000,000.0			13,700,000				11,000,000.0
Other Liabilities [Member]
Operating Lease, Liability	19,127,000			14,500,000				11,800,000
Retained Earnings [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 676,465,000	$ 662,311,000	$ 649,636,000	$ 646,847,000	$ 639,594,000	$ 625,227,000	$ 619,971,000	614,069,000
Accounting Standards Update 2016-02 [Member] | Retained Earnings [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest								613,926,000
Accounting Standards Update 2016-02 [Member] | Retained Earnings [Member] | Cumulative Effect, Period of Adoption, Adjustment
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest								$ (143,000)