Derivative Financial Instruments (Details 3) (Derivatives designated as cash flow hedging instruments, USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Derivatives designated as cash flow hedging instruments
Gain (loss) recognized in accumulated other comprehensive income, net of tax	$ 1,338	$ (2,160)
Gain (loss) reclassified from accumulated other comprehensive income (loss) to earnings	(616)	877
Natural gas hedges
Derivatives designated as cash flow hedging instruments
Gain (loss) recognized in accumulated other comprehensive income, net of tax		(2,132)
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	1,340	103
Gain (loss) reclassified from accumulated other comprehensive income (loss) to earnings	(616)	(565)
Interest rate cap
Derivatives designated as cash flow hedging instruments
Gain (loss) recognized in accumulated other comprehensive income, net of tax	$ (2)	$ (131)