Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net income (loss)	$ 87,351	$ 82,986	$ 165,891	$ (132,988)
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 $56 and $56 for the three months ended and $168 and $170 for the nine months ended in 2017 and 2016, respectively	92	92	275	275
Postretirement liability adjustment:
Amortization of postretirement liability (gains) losses included in net periodic benefit cost (credit), net of tax of $203 and $143 for the three months ended and $609 and $(676) for the nine months ended in 2017 and 2016, respectively	333	236	1,002	(1,111)
Reclassification of postretirement liability adjustment from regulatory asset, net of tax of $0 and $0 for the three months ended and $(725) and $0 for the nine months ended in 2017 and 2016, respectively	0	0	(917)	0
Postretirement liability adjustment	333	236	85	(1,111)
Foreign currency translation adjustment:
Foreign currency translation adjustment recognized during the period, net of tax of $9 and $(2) for the three months ended and $5 and $32 for the nine months ended in 2017 and 2016, respectively	15	(4)	9	52
Net unrealized gain (loss) on available-for-sale investments:
Net unrealized loss on available-for-sale investments arising during the period, net of tax of $(10) and $(23) for the three months ended and $(38) and $(35) for the nine months ended in 2017 and 2016, respectively	(19)	(42)	(70)	(65)
Reclassification adjustment for loss on available-for-sale investments included in net income (loss), net of tax of $14 and $18 for the three months ended and $50 and $57 for the nine months ended in 2017 and 2016, respectively	27	33	93	106
Net unrealized gain (loss) on available-for-sale investments	8	(9)	23	41
Other comprehensive income (loss)	448	315	392	(743)
Comprehensive income (loss)	87,799	83,301	166,283	(133,731)
Loss from discontinued operations attributable to noncontrolling interest	0	0	0	(131,691)
Comprehensive income (loss) attributable to common stockholders	$ 87,799	$ 83,301	$ 166,283	$ (2,040)