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Retirement Plans (Tables)
 6 Months Ended
Jun. 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 six months ended June 30, 2018 and 2017 were as follows.
 
Three Months Ended June 30,
Six Months Ended June 30,
 
2018
 
2017
2018
 
2017
 
($ in Thousands)
Components of Net Periodic Benefit Cost
 
 
 
 
 
 
RAP
 
 
 
 
 
 
Service cost
$
1,893

 
$
1,782

$
3,785

 
$
3,563

Interest cost
1,660

 
1,756

3,320

 
3,512

Expected return on plan assets
(4,755
)
 
(4,882
)
(9,510
)
 
(9,763
)
Amortization of prior service cost
(19
)
 
(18
)
(38
)
 
(37
)
Amortization of actuarial loss
463

 
487

925

 
975

Total net pension cost
$
(759
)
 
$
(875
)
$
(1,518
)
 
$
(1,750
)
Bank Mutual Pension Plan(a)
 
 
 
 
 
 
Interest cost
$
650

 
N/A

$
1,083

 
N/A

Expected return on plan assets
(1,060
)
 
N/A

(1,592
)
 
N/A

Total net pension cost
$
(410
)
 
N/A

$
(509
)
 
N/A

Postretirement Plan(b)
 
 
 
 
 
 
Interest cost
$
27

 
$
24

$
53

 
$
48

Amortization of prior service cost
(19
)
 
(19
)
(38
)
 
(38
)
Amortization of actuarial loss
2

 
—

4

 
—

Total net periodic benefit cost
$
10

 
$
5

$
19

 
$
10