Retirement Plans - Net Periodic Benefit Plan (Income) Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Defined Benefit Pension Plans
Net periodic benefit plan (income) cost included in non-interest expense
Interest cost	$ 946	$ 264	$ 1,892	$ 528
Return on plan assets	(772)	(137)	(1,729)	(274)
Net periodic benefit plan (income) cost	174	127	163	254
Postretirement Benefit Plans
Net periodic benefit plan (income) cost included in non-interest expense
Interest cost	35	30	71	60
Service cost	0	0	1	0
Amortization of prior service credit	(11)	(11)	(23)	(23)
Net periodic benefit plan (income) cost	$ 24	$ 19	$ 49	$ 37