Consolidated Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Consolidated Statements of Comprehensive Income [Abstract]
Net income (loss)	$ 46,333	$ 54,113	$ 102,848	$ 89,904
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 $52 and $15,059 for the three months ended and $(3,116) and $13,129 for the six months ended in 2013 and 2012, respectively	254	25,773	(5,594)	22,506
Reclassification adjustment for gain on derivative instruments included in net income, net of tax of $(322) and $(1,077) for the three months ended and $(1,948) and $(2,738) for the six months ended in 2013 and 2012, respectively	(396)	(1,834)	(3,168)	(4,666)
Net unrealized gain (loss) on derivative instruments qualifying as hedges	(142)	23,939	(8,762)	17,840
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 $(77) and $(23) for the three months ended and $(100) and $(26) for the six months ended in 2013 and 2012, respectively	(142)	(43)	(187)	(47)
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax of $23 and $20 for the three months ended and $42 and $37 for the six months ended in 2013 and 2012, respectively	44	38	79	68
Net unrealized gain (loss) on available-for-sale investments	(98)	(5)	(108)	21
Amortization of postretirement liability losses included in net periodic benefit cost, net of tax of $543 and $862 for the three and six months ended in 2013	424	0	1,072	0
Foreign currency translation adjustment, net of tax of $(234) and $(402) for the three months ended and $(197) and $(265) for the six months ended in 2013 and 2012, respectively	(390)	(579)	(302)	(435)
Other comprehensive income (loss)	(206)	23,355	(8,100)	17,426
Comprehensive income	46,127	77,468	94,748	107,330
Comprehensive loss attributable to noncontrolling interest	(179)	0	(179)	0
Comprehensive income attributable to common stockholders	$ 46,306	$ 77,468	$ 94,927	$ 107,330