Financial Derivatives - Narrative (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Derivative [Line Items]
Potential additional collateral to be pledged	$ 75,400,000		$ 75,400,000
Notional Amount	6,107,033,000		6,107,033,000		6,586,530,000
Counterparty level [Member]
Derivative [Line Items]
Fair value of derivative with counterparty	130,600,000		130,600,000
Basis Swaps | Zions First National Bank
Derivative [Line Items]
Notional Amount	49,300,000		49,300,000		72,700,000
Derivative fair value	(900,000)		(900,000)		(1,300,000)
Gains/(losses) on financial derivatives and hedging activities	100,000	(200,000)	500,000	1,400,000
Cash
Derivative [Line Items]
Securities pledged as collateral at fair value	$ 55,200,000		$ 55,200,000