Employee Benefits (Tables)
 9 Months Ended
Sep. 30, 2016
Retirement Plans  
Defined Benefit Plan Disclosure [Line Items]  
Schedule of Net Benefit Costs
The net periodic benefit cost recognized for the three, nine and twelve months ended September 30, 2016 and 2015 is made up of the components listed below as determined using the projected unit credit actuarial cost method (in thousands):
 
Three Months Ended
 
Nine Months Ended
 
Twelve Months Ended
 
September 30,
 
September 30,
 
September 30,
 
2016
 
2015
 
2016
 
2015
 
2016
 
2015
Components of net periodic benefit cost:
 
 
 
 
 
 
 
 
 
 
 
Service cost
$
2,191

 
$
2,394

 
$
6,001

 
$
6,594

 
$
8,199

 
$
8,707

Interest cost
3,250

 
3,622

 
9,780

 
10,872

 
13,403

 
14,563

Expected return on plan assets
(4,734
)
 
(4,951
)
 
(14,159
)
 
(14,846
)
 
(19,108
)
 
(19,528
)
Amortization of:
 
 
 
 
 
 
 
 
 
 
 
Net loss
1,730

 
2,485

 
5,505

 
7,985

 
8,167

 
10,267

Prior service benefit
(875
)
 
(855
)
 
(2,630
)
 
(2,630
)
 
(3,506
)
 
(3,506
)
Net periodic benefit cost
$
1,562

 
$
2,695

 
$
4,497

 
$
7,975

 
$
7,155

 
$
10,503
Other Postretirement Benefit Plans  
Defined Benefit Plan Disclosure [Line Items]  
Schedule of Net Benefit Costs
The net periodic benefit cost recognized for the three, nine and twelve months ended September 30, 2016 and 2015 is made up of the components listed below (in thousands): 
 
Three Months Ended
 
Nine Months Ended
 
Twelve Months Ended
 
September 30,
 
September 30,
 
September 30,
 
2016
 
2015
 
2016
 
2015
 
2016
 
2015
Components of net periodic benefit cost:
 
 
 
 
 
 
 
 
 
 
 
Service cost
$
749

 
$
841

 
$
2,179

 
$
2,591

 
$
3,042

 
$
3,302

Interest cost
871

 
976

 
2,616

 
3,026

 
3,625

 
4,142

Expected return on plan assets
(452
)
 
(503
)
 
(1,372
)
 
(1,553
)
 
(1,889
)
 
(2,082
)
Amortization of:
 
 
 
 
 
 
 
 
 
 
 
Prior service benefit
(788
)
 
(751
)
 
(2,363
)
 
(2,301
)
 
(3,130
)
 
(3,490
)
Net gain
(643
)
 
(519
)
 
(1,973
)
 
(1,519
)
 
(2,479
)
 
(2,186
)
Net periodic benefit cost (benefit)
$
(263
)
 
$
44

 
$
(913
)
 
$
244

 
$
(831
)
 
$
(314
)