Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Net income (loss)	$ 41,215	$ (229,804)	$ 78,539	$ (215,974)
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 $112 and $114 for the six months ended in 2017 and 2016, respectively	92	91	183	183
Postretirement liability adjustment:
Amortization of postretirement liability (gains) losses included in net periodic benefit cost (credit), net of tax of $190 and $150 for the three months ended and $406 and $(819) for the six months ended in 2017 and 2016, respectively	312	248	669	(1,347)
Reclassification of postretirement liability adjustment from regulatory asset, net of tax of $0 and $0 for the three months ended and $(725) and $0 in 2017 and 2016, respectively	0	0	(917)	0
Postretirement liability adjustment	312	248	(248)	(1,347)
Foreign currency translation adjustment:
Foreign currency translation adjustment recognized during the period, net of tax of $(9) and $19 for the three months ended and $(3) and $33 for the six months ended in 2017 and 2016, respectively	(15)	31	(6)	56
Net unrealized gain on available-for-sale investments:
Net unrealized loss on available-for-sale investments arising during the period, net of tax of $(13) and $(16) for the three months ended and $(28) and $(10) for the six months ended in 2017 and 2016, respectively	(24)	(30)	(51)	(19)
Reclassification adjustment for loss on available-for-sale investments included in net income (loss), net of tax of $17 and $19 for the three months ended and $36 and $37 for the six months ended in 2017 and 2016, respectively	31	36	66	69
Net unrealized gain on available-for-sale investments	7	6	15	50
Other comprehensive income (loss)	396	376	(56)	(1,058)
Comprehensive income (loss)	41,611	(229,428)	78,483	(217,032)
Loss from discontinued operations attributable to noncontrolling interest	0	(120,651)	0	(131,691)
Comprehensive income (loss) attributable to common stockholders	$ 41,611	$ (108,777)	$ 78,483	$ (85,341)