Derivative Financial Instruments (Details 3) (Derivatives designated as cash flow hedging instruments, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Derivatives designated as cash flow hedging instruments
Gain (loss), net of tax recognized in accumulated other comprehensive income (loss)	$ 4,886	$ (4,616)
(Gain) loss, net of tax, reclassified from accumulated other comprehensive income (loss) to earnings	(2,546)	1,200
Loss, net of tax, reclassified from accumulated other comprehensive income (loss) to earnings (ineffective portion)	184
Natural gas hedges
Derivatives designated as cash flow hedging instruments
Gain (loss), net of tax recognized in accumulated other comprehensive income (loss)		(5,812)
(Gain) loss, 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)	4,894	1,459
(Gain) loss, net of tax, reclassified from accumulated other comprehensive income (loss) to earnings	(2,546)	(2,079)
Loss, net of tax, reclassified from accumulated other comprehensive income (loss) to earnings (ineffective portion)	184
Interest rate cap
Derivatives designated as cash flow hedging instruments
Gain (loss), net of tax recognized in accumulated other comprehensive income (loss)	$ (8)	$ (263)