Derivative Financial Instruments (Details 3) (Derivatives designated as cash flow hedging instruments, USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Derivatives designated as cash flow hedging instruments
Gain (loss) recognized in accumulated other comprehensive income, net of tax	$ (27)	$ (1,909)	$ (5,551)	$ (2,528)
Gain (loss) reclassified from accumulated other comprehensive income to earnings, net of tax	(518)	475	1,736	381
Natural gas hedges
Derivatives designated as cash flow hedging instruments
Gain (loss) recognized in accumulated other comprehensive income, net of tax		464	(5,798)	1,245
Gain (loss) reclassified from accumulated other comprehensive income to earnings, net of tax		520	3,279	520
Interest rate swaps
Derivatives designated as cash flow hedging instruments
Gain (loss) recognized in accumulated other comprehensive income, net of tax	9	(4,008)	502	(4,068)
Gain (loss) reclassified from accumulated other comprehensive income to earnings, net of tax	(518)	(45)	(1,543)	(139)
Interest rate cap
Derivatives designated as cash flow hedging instruments
Gain (loss) recognized in accumulated other comprehensive income, net of tax	$ (36)	$ 1,635	$ (255)	$ 295