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Retirement Plans (Tables)
 9 Months Ended
Sep. 30, 2018
Defined Benefit Plan [Abstract]  
Net period benefit cost for the pension and postretirement plans The components of net periodic benefit cost for the RAP, Bank Mutual Pension Plan, and Postretirement Plan for three and nine months ended September 30, 2018 and 2017 were as follows.
 
Three Months Ended September 30,
Nine Months Ended September 30,
 
2018
 
2017
2018
 
2017
 
($ in Thousands)
Components of Net Periodic Benefit Cost
 
 
 
 
 
 
RAP
 
 
 
 
 
 
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 cost
$
(680
)
 
$
(821
)
$
(2,197
)
 
$
(2,571
)
Bank Mutual Pension Plan(a)
 
 
 
 
 
 
Interest cost
$
654

 
N/A

$
1,737

 
N/A

Expected return on plan assets
(1,220
)
 
N/A

(2,812
)
 
N/A

Total net pension cost
$
(566
)
 
N/A

$
(1,075
)
 
N/A

Postretirement Plan(b)
 
 
 
 
 
 
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 periodic benefit cost
$
11

 
$
9

$
30

 
$
19