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, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Derivatives designated as cash flow hedging instruments
Gain (loss), net of tax recognized in accumulated other comprehensive income (loss)	$ 1,054	$ (27)	$ 3,532	$ (5,551)
Gain, net of tax, reclassified from accumulated other comprehensive income (loss) to earnings	(636)	(518)	(1,866)	1,736
Loss, net of tax, reclassified from accumulated other comprehensive income (loss) to earnings (ineffective portion)	235		235
Natural gas hedges
Derivatives designated as cash flow hedging instruments
Gain (loss), net of tax recognized in accumulated other comprehensive income (loss)				(5,798)
Gain, net of tax, reclassified from accumulated other comprehensive income (loss) to earnings				3,279
Interest rate swaps
Derivatives designated as cash flow hedging instruments
Gain (loss), net of tax recognized in accumulated other comprehensive income (loss)	1,057	9	3,538	502
Gain, net of tax, reclassified from accumulated other comprehensive income (loss) to earnings	(636)	(518)	(1,866)	(1,543)
Loss, net of tax, reclassified from accumulated other comprehensive income (loss) to earnings (ineffective portion)	235		235
Interest rate cap
Derivatives designated as cash flow hedging instruments
Gain (loss), net of tax recognized in accumulated other comprehensive income (loss)	$ (3)	$ (36)	$ (6)	$ (255)