Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Period opening balance				$ (51,753)
Net current period other comprehensive income ending balance		$ 126	$ 171	527	$ 826
Period closing balance		(51,226)		(51,226)
Amortization of actuarial loss included in net periodic cost, before tax				(1,600)	(2,200)
Other Comprehensive (Income) Loss, Amortization Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, for Net Prior Service Cost (Credit), before Tax				800	800
Other comprehensive income (loss), tax				300	500
Defined Benefit Pension Plan Adjustments
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Period opening balance				(51,753)	(55,548)
Net current period other comprehensive income ending balance	[1]			527	826
Period closing balance		$ (51,226)	$ (54,722)	$ (51,226)	$ (54,722)

[1] For the six months ended June 30, 2017 and 2016, net periodic costs associated with our pension and other postretirement employee benefit, or OPEB, plans included in other comprehensive income and reclassified from accumulated other comprehensive loss were $1.6 million and $2.2 million, respectively, of actuarial loss amortization, as well as $0.8 million and $0.8 million, respectively, of prior service credit amortization, all net of tax totaling $0.3 million and $0.5 million, respectively. These accumulated other comprehensive loss components are included in the computation of net periodic pension and OPEB costs in Note 10, “Pension and Other Postretirement Employee Benefit Plans.”