Retirement Plans, Components of Net Periodic Benefit Cost for the Pension and Postretirement Tables (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
RAP
Net period benefit cost for the pension and postretirement plans
Service cost	$ 1,925	$ 1,893	$ 3,850	$ 3,785
Interest cost	2,413	1,660	4,825	3,320
Expected return on plan assets	(6,075)	(4,755)	(12,150)	(9,510)
Amortization of prior service cost	(19)	(19)	(38)	(38)
Amortization of actuarial loss	65	463	130	925
Total net pension and periodic benefit cost	(1,691)	(759)	(3,383)	(1,518)
Bank Mutual Pension Plan
Net period benefit cost for the pension and postretirement plans
Interest cost		650		1,083
Expected return on plan assets		(1,060)		(1,592)
Total net pension and periodic benefit cost		(410)		(509)
Postretirement Plan
Net period benefit cost for the pension and postretirement plans
Interest cost	26	27	52	53
Amortization of prior service cost	(19)	(19)	(38)	(38)
Amortization of actuarial loss	(1)	2	(2)	4
Total net pension and periodic benefit cost	$ 6	$ 10	$ 13	$ 19