Retirement Plans - Net Periodic Benefit Plan (Income) Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Pension Plans
Net periodic benefit plan (income) cost included in non-interest expense
Interest cost	$ 946	$ 1,046	$ 2,839	$ 1,574
Return on plan assets	(864)	(949)	(2,594)	(1,223)
Net periodic benefit plan (income) cost	82	97	245	351
Postretirement Benefit Plans
Net periodic benefit plan (income) cost included in non-interest expense
Interest cost	36	41	107	101
Service cost	0	0	1	0
Amortization of prior service credit	(12)	(11)	(35)	(35)
Net periodic benefit plan (income) cost	$ 24	$ 30	$ 73	$ 66