Consoldiated Statements of Comprehensive Income (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net income (loss)	$ (754,000)	$ 213,026,000	$ 240,659,000
Net unrealized gain (loss) on derivative instruments arising during the period, net of tax of $4,829, $4,683 and $(1,867) in 2012, 2011 and 2010, respectively	8,497,000	7,900,000	(3,077,000)
Less: Reclassification adjustment for gain (loss) on derivative instruments included in net income, net of tax of $5,141, $0 and $(2,305) in 2012, 2011 and 2010, respectively	8,754,000	0	(3,750,000)
Net unrealized gain (loss) on derivative instruments qualifying as hedges	(257,000)	7,900,000	673,000
Postretirement liability losses arising during the period, net of tax of $(2,060), $(14,205) and $(4,112) in 2012, 2011 and 2010, respectively	(3,106,000)	(23,473,000)	(6,528,000)
Amortization of postretirement liability losses included in net periodic benefit cost, net of tax of $1,379, $632 and $503 in 2012, 2011 and 2010, respectively	2,079,000	1,046,000	798,000
Postretirement liability adjustment	(1,027,000)	(22,427,000)	(5,730,000)
Foreign currency translation adjustment, net of tax of $(294), $(832) and $(3,486) in 2012, 2011 and 2010, respectively	(473,000)	(1,295,000)	(5,371,000)
Net unrealized gains on available-for-sale investments, net of tax of $20 and $44 in 2012 and 2011, respectively	37,000	82,000	0
Other comprehensive loss	(1,720,000)	(15,740,000)	(10,428,000)
Comprehensive income (loss)	(2,474,000)	197,286,000	230,231,000
Other Comprehensive Income (Loss), Tax, Parenthetical Disclosures [Abstract]
Net unrealized gain (loss) on derivative instruments arising during the period, tax	4,829,000	4,683,000	(1,867,000)
Less: Reclassification adjustment for gain (loss) on derivative instruments included in net income, tax	5,141,000	0	(2,305,000)
Postretirement liability losses arising during the period, tax	(2,060,000)	(14,205,000)	(4,112,000)
Amortization of postretirement liability losses included in net periodic benefit cost, tax	1,379,000	632,000	503,000
Foreign currency translation adjustment, tax	(294,000)	(832,000)	(3,486,000)
Net unrealized gains on available-for-sale investments, tax	$ 20,000	$ 44,000	$ 0