Derivative Instruments and Hedging Activities (Components of Gain (Loss) on Derivative Instruments) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Unrealized gain due to change in fair market value	$ 34,683	$ 30,500	$ 27,300	$ 13,958
Realized gain (loss) on settlement	2,247	(195)	(4,321)	(3,715)
Total gain on derivative instruments	36,930	30,305	22,979	10,243
Parent Company [Member]
Total gain on derivative instruments	$ 36,930	$ 30,305	$ 22,979	$ 10,243