Retirement Plans, Components of Net Periodic Benefit Cost for the Pension and Postretirement Tables (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
RAP
Net period benefit cost for the pension and postretirement plans
Service cost	$ 1,885	$ 1,713	$ 5,670	$ 5,276
Interest cost	1,682	1,795	5,002	5,307
Expected return on plan assets	(4,777)	(4,929)	(14,287)	(14,692)
Amortization of prior service cost	(18)	(19)	(56)	(56)
Amortization of actuarial loss	549	619	1,474	1,594
Total net pension and periodic benefit cost	(680)	(821)	(2,197)	(2,571)
Bank Mutual Pension Plan
Net period benefit cost for the pension and postretirement plans
Interest cost	654		1,737
Expected return on plan assets	(1,220)		(2,812)
Total net pension and periodic benefit cost	(566)		(1,075)
Postretirement Plan(b)
Net period benefit cost for the pension and postretirement plans
Interest cost	27	26	80	74
Amortization of prior service cost	(19)	(19)	(56)	(57)
Amortization of actuarial loss	2	2	6	2
Total net pension and periodic benefit cost	$ 11	$ 9	$ 30	$ 19