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Components of net periodic benefit cost (Tables)
 6 Months Ended
Jun. 30, 2014
Components of net periodic benefit cost [Abstract]  
Schedule of net periodic benefit cost
 
Pension Benefits
 
Three months ended June 30,

Six months ended June 30,
 
2014
2013

2014
2013
Service cost
$
1,079

$
926

 
$
2,666

$
1,606

Interest cost
3,101

1,755

 
5,741

3,390

Expected return on plan assets
(3,701
)
(1,893
)
 
(7,098
)
(3,613
)
Amortization of prior service costs
9

22

 
18

54

Amortization of net loss
167

857

 
953

1,706

Curtailment
—

(18
)
 
—

(18
)
Net periodic benefit cost
$
655

$
1,649

 
$
2,280

$
3,125

 
Other Postretirement Benefits ("OPEB")
 
Three months ended June 30,
 
Six months ended June 30,
 
2014
2013
 
2014
2013
Service cost
$
117

$
670

 
$
794

$
1,163

Interest cost
1,695

1,433

 
3,206

2,756

Amortization of prior service cost
(961
)
(938
)
 
(1,922
)
(1,998
)
Amortization of net loss
828

1,273

 
1,852

2,776

Curtailment
—

(20
)
 
—

(20
)
Net periodic benefit cost
$
1,679

$
2,418

 
$
3,930

$
4,677