Derivative Financial Instruments (Details 3) (Derivatives designated as cash flow hedging instruments, USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Derivatives designated as cash flow hedging instruments
Gain (loss) recognized in accumulated other comprehensive income, net of tax	$ (2,160)	$ (72)
Gain (loss) reclassified from accumulated other comprehensive income (loss) to earnings	877	(46)
Natural gas hedges
Derivatives designated as cash flow hedging instruments
Gain (loss) recognized in accumulated other comprehensive income, net of tax	(2,132)	(167)
Gain (loss) reclassified from accumulated other comprehensive income (loss) to earnings	1,442
Interest rate swaps
Derivatives designated as cash flow hedging instruments
Gain (loss) recognized in accumulated other comprehensive income, net of tax	103	95
Gain (loss) reclassified from accumulated other comprehensive income (loss) to earnings	(565)	(46)
Interest rate cap
Derivatives designated as cash flow hedging instruments
Gain (loss) recognized in accumulated other comprehensive income, net of tax	$ (131)