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Employee Benefits (Tables)
 3 Months Ended
Mar. 31, 2018
Retirement Plans  
Defined Benefit Plan Disclosure [Line Items]  
Schedule of Net Benefit Costs
The net periodic benefit cost recognized for the three and twelve months ended March 31, 2018 and 2017, is made up of the components listed below as determined using the projected unit credit actuarial cost method (in thousands):
 
Three Months Ended
 
Twelve Months Ended
 
March 31,
 
March 31,
 
2018
 
2017
 
2018
 
2017
Components of net periodic benefit cost:
 
 
 
 
 
 
 
Service cost
$
2,758

 
$
2,270

 
$
9,006

 
$
8,366

Interest cost
3,223

 
3,248

 
13,034

 
13,022

Expected return on plan assets
(5,315
)
 
(4,808
)
 
(19,696
)
 
(18,975
)
Amortization of:
 
 
 
 
 
 
 
Net loss
2,100

 
2,089

 
8,465

 
7,540

Prior service benefit
(878
)
 
(878
)
 
(3,506
)
 
(3,506
)
Net periodic benefit cost
$
1,888

 
$
1,921

 
$
7,303

 
$
6,447
Other Postretirement Benefit Plans  
Defined Benefit Plan Disclosure [Line Items]  
Schedule of Net Benefit Costs
The net periodic benefit recognized for the three and twelve months ended March 31, 2018 and 2017, is made up of the components listed below (in thousands): 
 
Three Months Ended
 
Twelve Months Ended
 
March 31,
 
March 31,
 
2018
 
2017
 
2018
 
2017
Components of net periodic benefit:
 
 
 
 
 
 
 
Service cost
$
700

 
$
588

 
$
2,348

 
$
2,642

Interest cost
565

 
678

 
2,610

 
2,972

Expected return on plan assets
(613
)
 
(470
)
 
(2,050
)
 
(1,845
)
Amortization of:
 
 
 
 
 
 
 
Prior service benefit
(1,538
)
 
(1,538
)
 
(6,151
)
 
(4,651
)
Net gain
(525
)
 
(395
)
 
(1,808
)
 
(2,104
)
Net periodic benefit
$
(1,411
)
 
$
(1,137
)
 
$
(5,051
)
 
$
(2,986
)