Retirement Plans - Net Periodic Benefit Plan (Income) Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plan
Net periodic benefit plan (income) cost included in other non-interest expense
Interest cost	$ 3,785	$ 3,013	$ 983
Service cost	0	0	0
Return on plan assets	(12,551)	(333)	607
Recognized actuarial loss (gain)	782	3,831	(630)
Amortization of prior service credit (recognized in net periodic benefit cost)	0	0	0
Net periodic benefit plan (income) cost	(7,984)	6,511	960
Employer contributions	2,423	0	0
Postretirement Plan
Net periodic benefit plan (income) cost included in other non-interest expense
Interest cost	143	149	110
Service cost	1	1	0
Return on plan assets	0	0	0
Recognized actuarial loss (gain)	295	(19)	(115)
Amortization of prior service credit (recognized in net periodic benefit cost)	(46)	(46)	(46)
Net periodic benefit plan (income) cost	393	85	(51)
Employer contributions	$ 460	$ 377	$ 392