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		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Fair Value Measurements [Abstract]
Balance at beginning of period	$ (60,147)	$ 184,543	$ (115)	$ 182,119
Total gains or losses (realized/unrealized): Included in earnings	32,277	65,274	(26,748)	123,736
Total gains or losses (realized/unrealized): Included in other comprehensive income		(77,731)		(77,441)
Purchases, issuances, and settlements:
Purchases
Issuances
Settlements	(88)	(65,864)	(1,095)	(122,192)
Balance at end of period	(27,958)	106,222	(27,958)	106,222
Change in unrealized gain included in earnings relating to derivatives still held as of June 30	$ 32,189	$ (590)	$ (27,843)	$ 1,544