Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net income (loss)	$ (657,691)	$ 294,338	$ 278,570
Net unrealized gain (loss) on derivative instruments qualifying as hedges:
Net unrealized loss on derivative instruments arising during the period, net of tax of $0, $0 and $(3,116) in 2015, 2014 and 2013, respectively	0	0	(5,594)
Reclassification adjustment for loss on derivative instruments included in net income (loss), net of tax of $233, $240 and $339 in 2015, 2014 and 2013, respectively	404	399	727
Reclassification adjustment for (gain) loss on derivative instruments included in income (loss) from discontinued operations, net of tax of $0, $173 and $(2,887) in 2015, 2014 and 2013, respectively	0	295	(4,916)
Net unrealized gain (loss) on derivative instruments qualifying as hedges	404	694	(9,783)
Postretirement liability adjustment:
Postretirement liability gains (losses) arising during the period, net of tax of $(55), $(7,665) and $11,818 in 2015, 2014 and 2013, respectively	(88)	(12,409)	18,539
Amortization of postretirement liability losses included in net periodic benefit cost (credit), net of tax of $1,128, $492 and $1,276 in 2015, 2014 and 2013, respectively	1,794	796	2,001
Reclassification of postretirement liability adjustment to regulatory asset, net of tax of $1,416, $4,509 and $0 in 2015, 2014 and 2013, respectively	2,255	7,202	0
Postretirement liability adjustment	3,961	(4,411)	20,540
Foreign currency translation adjustment:
Foreign currency translation adjustment recognized during the period, net of tax of $(105), $(99) and $(177) in 2015, 2014 and 2013, respectively	(173)	(162)	(299)
Reclassification adjustment for loss on foreign currency translation adjustment included in net income (loss), net of tax of $490, $0 and $70 in 2015, 2014 and 2013, respectively	802	0	143
Foreign currency translation adjustment	629	(162)	(156)
Net unrealized loss on available-for-sale investments:
Net unrealized loss on available-for-sale investments arising during the period, net of tax of $(91), $(83) and $(105) in 2015, 2014 and 2013, respectively	(170)	(154)	(194)
Reclassification adjustment for loss on available-for-sale investments included in net income (loss), net of tax of $70, $73 and $59 in 2015, 2014 and 2013, respectively	131	135	109
Net unrealized loss on available-for-sale investments	(39)	(19)	(85)
Other comprehensive income (loss)	4,955	(3,898)	10,516
Comprehensive income (loss)	(652,736)	290,440	289,086
Comprehensive loss attributable to noncontrolling interest	(35,256)	(3,895)	(363)
Comprehensive income (loss) attributable to common stockholders	$ (617,480)	$ 294,335	$ 289,449