Retirement Plans, Components of Net Periodic Benefit Cost for the Pension and Postretirement Tables (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
RAP
Net period benefit cost for the pension and postretirement plans
Service cost	$ 1,925	$ 1,893
Interest cost	2,413	1,660
Expected return on plan assets	(6,075)	(4,755)
Amortization of prior service cost	(19)	(19)
Amortization of actuarial loss	65	463
Total net pension and periodic benefit cost	(1,691)	(759)
Bank Mutual Pension Plan
Net period benefit cost for the pension and postretirement plans
Interest cost		433
Expected return on plan assets		(532)
Total net pension and periodic benefit cost		(99)
Postretirement Plan
Net period benefit cost for the pension and postretirement plans
Interest cost	26	26
Amortization of prior service cost	(19)	(19)
Amortization of actuarial loss	(1)	2
Total net pension and periodic benefit cost	$ 6	$ 9