Components of net periodic benefit cost (Tables)

9 Months Ended

Sep. 30, 2014

Components of net periodic benefit cost [Abstract]

Schedule of net periodic benefit cost

	Pension Benefits
	Three months ended September 30,						Nine months ended September 30,
	2014			2013			2014			2013
Service cost	$	1,333		$	1,686		$	3,999		$	3,292
Interest cost	2,871			2,903			8,612			6,293
Expected return on plan assets	(3,550		)	(3,723		)	(10,648		)	(7,337		)
Amortization of prior service costs	9			30			27			85
Amortization of net loss	477			723			1,430			2,428
Curtailment	—			—			—			(18		)
Net periodic benefit cost	$	1,140		$	1,619		$	3,420		$	4,743

	Other Postretirement Benefits ("OPEB")
	Three months ended September 30,						Nine months ended September 30,
	2014			2013			2014			2013
Service cost	$	397		$	700		$	1,191		$	1,862
Interest cost	1,603			1,474			4,809			4,230
Amortization of prior service cost	(961		)	(998		)	(2,883		)	(2,997		)
Amortization of net loss	925			1,157			2,777			3,934
Curtailment	—			—			—			(20		)
Net periodic benefit cost	$	1,964		$	2,333		$	5,894		$	7,009