Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net income (loss)	$ 281,203	$ (67,258)	$ (657,691)
Net unrealized gain on derivative instruments qualifying as hedges:
Reclassification adjustment for loss on derivative instruments included in net income (loss), net of tax of $224, $226 and $233 in 2017, 2016 and 2015, respectively	366	367	404
Postretirement liability adjustment:
Postretirement liability losses arising during the period, net of tax of $(1,162), $(836) and $(55) in 2017, 2016 and 2015, respectively	(1,812)	(1,470)	(88)
Amortization of postretirement liability losses included in net periodic benefit cost (credit), net of tax of $645, $1,425 and $1,128 in 2017, 2016 and 2015, respectively	1,013	2,506	1,794
Reclassification of postretirement liability adjustment (from) to regulatory asset, net of tax of $(876), $0 and $1,416 in 2017, 2016 and 2015, respectively	(1,143)	0	2,255
Postretirement liability adjustment	(1,942)	1,036	3,961
Foreign currency translation adjustment:
Foreign currency translation adjustment recognized during the period, net of tax of $(3), $31 and $(105) in 2017, 2016 and 2015, respectively	(6)	51	(173)
Reclassification adjustment for loss on foreign currency translation adjustment included in net income (loss), net of tax of $0, $0 and $490 in 2017, 2016 and 2015, respectively	0	0	802
Foreign currency translation adjustment	(6)	51	629
Net unrealized loss on available-for-sale investments:
Net unrealized loss on available-for-sale investments arising during the period, net of tax of $(75), $(98) and $(91) in 2017, 2016 and 2015, respectively	(139)	(182)	(170)
Reclassification adjustment for loss on available-for-sale investments included in net income (loss), net of tax of $65, $77 and $70 in 2017, 2016 and 2015, respectively	120	143	131
Net unrealized loss on available-for-sale investments	(19)	(39)	(39)
Other comprehensive income (loss)	(1,601)	1,415	4,955
Comprehensive income (loss)	279,602	(65,843)	(652,736)
Loss from discontinued operations attributable to noncontrolling interest	0	(131,691)	(35,256)
Comprehensive income (loss) attributable to common stockholders	$ 279,602	$ 65,848	$ (617,480)