Consolidated Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Consolidated Statements of Comprehensive Income [Abstract]
Net income	$ 53,327	$ 46,333	$ 109,466	$ 102,848
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 $0 and $52 for the three months ended and $0 and $(3,116) for the six months ended in 2014 and 2013, respectively	0	254	0	(5,594)
Reclassification adjustment for (gain) loss on derivative instruments included in net income, net of tax of $10 and $(322) for the three months ended and $213 and $(1,948) for the six months ended in 2014 and 2013, respectively	13	(395)	357	(3,168)
Net unrealized gain (loss) on derivative instruments qualifying as hedges	13	(141)	357	(8,762)
Amortization of postretirement liability losses included in net periodic benefit cost, net of tax of $150 and $543 for the three months ended and $318 and $862 for the six months ended in 2014 and 2013, respectively	245	424	520	1,072
Foreign currency translation adjustment recognized during the period, net of tax of $26 and $(234) for the three months ended and $54 and $(197) for the six months ended in 2014 and 2013, respectively	42	(390)	88	(302)
Net unrealized gain (loss) on available-for-sale investments:
Net unrealized gain (loss) on available-for-sale investments arising during the period, net of tax of $4 and $(77) for the three months ended and $5 and $(100) for the six months ended in 2014 and 2013, respectively	8	(142)	10	(187)
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax of $17 and $23 for the three months ended and $17 and $42 for the six months ended in 2014 and 2013, respectively	32	43	32	79
Net unrealized gain (loss) on available-for-sale investments	40	(99)	42	(108)
Other comprehensive income (loss)	340	(206)	1,007	(8,100)
Comprehensive income	53,667	46,127	110,473	94,748
Comprehensive loss attributable to noncontrolling interest	(779)	(179)	(1,302)	(179)
Comprehensive income attributable to common stockholders	$ 54,446	$ 46,306	$ 111,775	$ 94,927