Consolidated Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Consolidated Statements of Comprehensive Income [Abstract]
Net income (loss)	$ 56,515	$ 35,790
Net unrealized loss on derivative instruments qualifying as hedges:
Net unrealized loss on derivative instruments arising during the period, net of tax of $(3,168) and $(2,225) in 2013 and 2012, respectively	(5,849)	(3,770)
Less: Reclassification adjustment for gain on derivative instruments included in net income, net of tax	2,772	2,329
Net unrealized loss on derivative instruments qualifying as hedges	(8,621)	(6,099)
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 $(24) and $(2) in 2013 and 2012, respectively	(44)	(4)
Less: Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax of $(19) and $(16) in 2013 and 2012, respectively	(35)	(30)
Net unrealized gain (loss) on available-for-sale investments	(9)	26
Amortization of postretirement liability losses included in net periodic benefit cost, net of tax	648	0
Foreign currency translation adjustment, net of tax of $37 and $138 in 2013 and 2012, respectively	88	144
Other comprehensive loss	(7,894)	(5,929)
Comprehensive income	$ 48,621	$ 29,861