Retirement Plans - Net Periodic Benefit Plan (Income) Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Defined Benefit Pension Plans
Net periodic benefit plan (income) cost included in non-interest expense
Interest cost	$ 946	$ 264
Return on plan assets	(957)	(137)
Net periodic benefit plan (income) cost	(11)	127
Postretirement Benefit Plans
Net periodic benefit plan (income) cost included in non-interest expense
Interest cost	36	30
Service cost	1	0
Amortization of prior service credit		(12)
Amortization of unrecognized net loss	(12)
Net periodic benefit plan (income) cost	$ 25	$ 18