Significant Accounting Policies - Impact of new financial accounting standards (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Recently Issued Accounting Standards
Change in Accounting Principle, Accounting Standards Update, Adopted [true false]			true
Stockholders' Equity Attributable to Parent	$ 1,255,705	$ 1,236,084		$ 1,220,434	$ 1,215,981	$ 1,203,608	$ 994,964
Right of use asset	7,251			9,490
Lease liability	7,342			9,552
Retained Earnings (Accumulated Deficit)	4,718			$ (14,813)
Cumulative Effect, Period of Adoption, Adjustment
Recently Issued Accounting Standards
Stockholders' Equity Attributable to Parent	$ (15,922)
Retained Earnings (Accumulated Deficit)			$ (15,900)
Cumulative Effect, Period of Adoption, Adjustment | Allowance for credit losses on loans
Recently Issued Accounting Standards
Retained Earnings (Accumulated Deficit)			(12,000)
Cumulative Effect, Period of Adoption, Adjustment | Reserve for off-balance-sheet credit exposures
Recently Issued Accounting Standards
Retained Earnings (Accumulated Deficit)			$ (3,900)