Financial Derivatives Narrative (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Derivative [Line Items]
Additional Collateral, Aggregate Fair Value	$ 75,500,000
Derivative, Notional Amount	6,815,268,000	6,586,530,000
Excluding netting [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	37,100,000	46,600,000
Netting [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	2,400,000	4,600,000
Counterparty level [Member]
Derivative [Line Items]
Derivative, Net Liability Position, Aggregate Fair Value	135,800,000
Cash [Member]
Derivative [Line Items]
Collateral Already Posted, Aggregate Fair Value	60,300,000
Zions First National Bank [Member] | Basis Swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	49,300,000	72,700,000
Derivative, Fair Value, Net	(700,000)	(1,300,000)
Gain (Loss) on Derivative Instruments, Net, Pretax	$ 600,000	$ 2,100,000	$ 300,000