Consolidiated Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Net income (loss)	$ 278,570	$ (754)	$ 213,026
Net unrealized gain (loss) on derivative instruments qualifying as hedges:
Net unrealized gain (loss) on derivative instruments arising during the period, net of tax of $(3,116), $4,829 and $4,683 in 2013, 2012 and 2011, respectively	(5,594)	8,497	7,900
Reclassification adjustment for (gain) loss on derivative instruments included in net income, net of tax of $(2,548), $(5,141) and $0 in 2013, 2012 and 2011, respectively	(4,189)	(8,754)	0
Net unrealized gain (loss) on derivative instruments qualifying as hedges	(9,783)	(257)	7,900
Postretirement liability adjustment:
Postretirement liability gains (losses) arising during the period, net of tax of $11,818, $(2,060) and $(14,205) in 2013, 2012 and 2011, respectively	18,539	(3,106)	(23,473)
Amortization of postretirement liability losses included in net periodic benefit cost, net of tax of $1,276, $1,379 and $632 in 2013, 2012 and 2011, respectively	2,001	2,079	1,046
Postretirement liability adjustment	20,540	(1,027)	(22,427)
Foreign currency translation adjustment:
Foreign currency translation adjustment recognized during the period, net of tax of $(177), $(296) and $(767) in 2013, 2012 and 2011, respectively	(299)	(476)	(1,189)
Reclassification adjustment for (gain) loss on foreign currency translation adjustment included in net income, net of tax of $70, $2 and $(65) in 2013, 2012 and 2011, respectively	143	3	(106)
Foreign currency translation adjustment	(156)	(473)	(1,295)
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 $(105), $(52) and $(20) in 2013, 2012 and 2011, respectively	(194)	(97)	(36)
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax of $59, $72 and $64 in 2013, 2012 and 2011, respectively	109	134	118
Net unrealized gain (loss) on available-for-sale investments	(85)	37	82
Other comprehensive income (loss)	10,516	(1,720)	(15,740)
Comprehensive income (loss)	289,086	(2,474)	197,286
Comprehensive loss attributable to noncontrolling interest	(363)	0	0
Comprehensive income (loss) attributable to common stockholders	289,449	(2,474)	197,286
Other Comprehensive Income (Loss), Tax, Parenthetical Disclosures [Abstract]
Net unrealized gain (loss) on derivative instruments arising during the period, tax	(3,116)	4,829	4,683
Reclassification adjustment for (gain) loss on derivative instruments included in net income, tax	(2,548)	(5,141)	0
Postretirement liability gains (losses) arising during the period, tax	11,818	(2,060)	(14,205)
Amortization of postretirement liability losses included in net periodic benefit cost, tax	1,276	1,379	632
Foreign currency translation adjustment recognized during the period, tax	(177)	(296)	(767)
Reclassification adjustment for (gain) loss on foreign currency translation adjustment included in net income, tax	70	2	(65)
Net unrealized gains (losses) on available-for-sale investments arising during the period, tax	(105)	(52)	(20)
Reclassification adjustments for (gain) loss on available-for-sale investments included in net income, tax	$ 59	$ 72	$ 64