Consolidated Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Consolidated Statements of Comprehensive Income [Abstract]
Net income	$ 102,121	$ 84,432	$ 211,588	$ 187,280
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 and $0 for the three months ended and $0 and $(3,116) for the nine months ended in 2014 and 2013, respectively	0	0	0	(5,594)
Reclassification adjustment for (gain) loss on derivative instruments included in net income, net of tax of $50 and $(297) for the three months ended and $264 and $(2,246) for the nine months ended in 2014 and 2013, respectively	82	(510)	439	(3,678)
Net unrealized gain (loss) on derivative instruments qualifying as hedges	82	(510)	439	(9,272)
Amortization of postretirement liability losses included in net periodic benefit cost, net of tax of $159 and $166 for the three months ended and $477 and $1,027 for the nine months ended in 2014 and 2013, respectively	261	271	781	1,344
Foreign currency translation adjustment recognized during the period, net of tax of $(89) and $(12) for the three months ended and $(36) and $(209) for the nine months ended in 2014 and 2013, respectively	(146)	(20)	(58)	(351)
Reclassification adjustment for loss on foreign currency translation adjustment included in net income, net of tax of $0 and $70 for the three months ended and $0 and $70 for the nine months ended in 2014 and 2013, respectively	0	115	0	143
Foreign currency translation adjustment	(146)	95	(58)	(208)
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 $(33)and $(5) for the three months ended and $(48) and $(106) for the nine months ended in 2014 and 2013, respectively	(62)	(10)	(89)	(197)
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax of $16 and $20 for the three months ended and $54 and $63 for the nine months ended in 2014 and 2013, respectively	31	38	100	117
Net unrealized gain (loss) on available-for-sale investments	(31)	28	11	(80)
Other comprehensive income (loss)	166	(116)	1,173	(8,216)
Comprehensive income	102,287	84,316	212,761	179,064
Comprehensive loss attributable to noncontrolling interest	(1,088)	(24)	(2,390)	(204)
Comprehensive income attributable to common stockholders	$ 103,375	$ 84,340	$ 215,151	$ 179,268