Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net income (loss)	$ 272,318	$ 281,203	$ (67,258)
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 $429, $224 and $226 in 2018, 2017 and 2016, respectively	162	366	367
Postretirement liability adjustment:
Postretirement liability gains (losses) arising during the period, net of tax of $1,471, $(1,162) and $(836) in 2018, 2017 and 2016, respectively	4,441	(1,812)	(1,470)
Amortization of postretirement liability losses included in net periodic benefit cost (credit), net of tax of $721, $645 and $1,425 in 2018, 2017 and 2016, respectively	2,173	1,013	2,506
Reclassification of postretirement liability adjustment from regulatory asset, net of tax of $0, $(876) and $0 in 2018, 2017 and 2016, respectively	0	(1,143)	0
Postretirement liability adjustment	6,614	(1,942)	1,036
Foreign currency translation adjustment:
Foreign currency translation adjustment recognized during the period, net of tax of $(14), $(3) and $31 in 2018, 2017 and 2016, respectively	(61)	(6)	51
Reclassification adjustment for foreign currency translation adjustment included in net income (loss), net of tax of $75, $0 and $0 in 2018, 2017 and 2016, respectively	249	0	0
Foreign currency translation adjustment	188	(6)	51
Net unrealized loss on available-for-sale investments:
Net unrealized loss on available-for-sale investments arising during the period, net of tax of $(38), $(75) and $(98) in 2018, 2017 and 2016, respectively	(144)	(139)	(182)
Reclassification adjustment for loss on available-for-sale investments included in net income (loss), net of tax of $35, $65 and $77 in 2018, 2017 and 2016, respectively	131	120	143
Net unrealized loss on available-for-sale investments	(13)	(19)	(39)
Other comprehensive income (loss)	6,951	(1,601)	1,415
Comprehensive income (loss)	279,269	279,602	(65,843)
Loss from discontinued operations attributable to noncontrolling interest	0	0	(131,691)
Comprehensive income attributable to common stockholders	$ 279,269	$ 279,602	$ 65,848