Derivative Instruments and Hedging Activities (Components of Gain (Loss) on Derivative Instruments) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Unrealized gain (loss) due to change in fair market value	$ (9,491)	$ 11,475	$ 3,500
Realized loss on settlement	(3,814)	(17,345)	(8,365)
Total gain (loss) on derivative instruments	(13,305)	(5,870)	(4,865)
Parent Company [Member]
Total gain (loss) on derivative instruments	$ (13,305)	$ (5,870)	$ (4,865)