Basis of Presentation (Details) $ in Thousands		3 Months Ended
	Jan. 01, 2020 USD ($)	Mar. 31, 2020 USD ($) segment subsidiary	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Jan. 01, 2019 USD ($)
Basis of presentation
Number of operating segments | segment		1
Number of active operating bank subsidiaries | subsidiary		5
Right-of-use asset amortization		$ 320	$ 214
Retained earnings		$ 2,191,946		$ 2,200,568
ASU 2016-02
Basis of presentation
Right-of-use asset					$ 6,400
Statement of Financial Position Location					us-gaap:OtherAssets
Lease liability					$ 6,400
Statement of Financial Position Location					us-gaap:OtherLiabilities
ASU 2016-13 | Restatement Adjustment
Basis of presentation
Percentage of allowance for loan losses	17.20%
Retained earnings	$ (8,300)