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Postretirement benefits
 9 Months Ended
Sep. 30, 2016
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]  
Postretirement benefits
Postretirement benefits

We have historically provided certain health care benefits for a portion of our retired U.S. employees. In addition to our retiree health care plan, we also have a supplemental executive retirement plan in the United States. Further information regarding our postretirement benefit plans can be found under the caption “Note 12: Postretirement benefits” in the Notes to Consolidated Financial Statements appearing in the 2015 Form 10-K.

Postretirement benefit income for each period consisted of the following components:
 
 
Quarter Ended
September 30,
 
Nine Months Ended
September 30,
(in thousands)
 
2016
 
2015
 
2016
 
2015
Interest cost
 
$
780

 
$
859

 
$
2,339

 
$
2,578

Expected return on plan assets
 
(1,834
)
 
(1,958
)
 
(5,501
)
 
(5,875
)
Amortization of prior service credit
 
(355
)
 
(355
)
 
(1,066
)
 
(1,066
)
Amortization of net actuarial losses
 
949

 
780

 
2,848

 
2,340

Net periodic benefit income
 
$
(460
)
 
$
(674
)
 
$
(1,380
)
 
$
(2,023
)