Fair Value Measurements (Assets and Liabilities Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3), Reconciliation) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Fair Value Measurements [Abstract]
Balance at beginning of period	$ (115)	$ 182,119
Total gains or losses (realized/unrealized): Included in earnings	(59,025)	58,462
Total gains or losses (realized/unrealized): Included in other comprehensive income		290
Purchases, issuances, and settlements:
Purchases
Issuances
Settlements	(1,007)	(56,328)
Balance at end of period	(60,147)	184,543
Balance at beginning of period	(60,147)	184,543
Change in unrealized gain (loss) included in earnings relating to derivatives still held as of March 31	$ (60,032)	$ 2,134