Financial Derivatives - Narrative (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Derivative [Line Items]
Potential additional collateral to be pledged	$ 74,000,000		$ 74,000,000
Losses on financial derivatives	(31,292,000)	(17,806,000)	(24,892,000)	(13,801,000)
Counterparty level [Member]
Derivative [Line Items]
Fair value of derivative with counterparty	142,600,000		142,600,000
Basis Swaps | Zions First National Bank
Derivative [Line Items]
Notional Amount	60,100,000		60,100,000		72,700,000
Derivative fair value	(1,000,000)		(1,000,000)		(1,300,000)
Losses on financial derivatives	300,000	(100,000)	400,000	1,600,000
Cash
Derivative [Line Items]
Securities pledged as collateral at fair value	$ 68,600,000		$ 68,600,000