Basis of Presentation (Details) $ in Thousands		3 Months Ended		6 Months Ended
	Jan. 01, 2020 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) subsidiary segment	Jun. 30, 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		$ 338	$ 245	$ 658	$ 459
Retained earnings		$ 2,233,455		$ 2,233,455		$ 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