Financial Derivatives - Narrative (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Derivative [Line Items]
Derivative, Notional Amount	$ 6,592,077,000		$ 6,592,077,000		$ 6,815,268,000
Financial derivatives, at fair value	91,445,000		91,445,000		150,682,000
Collateral Already Posted, Aggregate Fair Value	11,977,000		11,977,000		60,311,000
Additional Collateral, Aggregate Fair Value	64,000,000		64,000,000		75,500,000
Uncollateralized [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	3,000,000		3,000,000		800,000
Cash [Member]
Derivative [Line Items]
Obligation to Return Securities Received as Collateral	742,000		742,000		1,650,000
Collateral Already Posted, Aggregate Fair Value	10,500,000		10,500,000		60,300,000
Securities Pledged as Collateral [Member]
Derivative [Line Items]
Collateral Already Posted, Aggregate Fair Value	1,500,000		1,500,000
Dedesignated hedge [Member]
Derivative [Line Items]
Derivative, Number of Instruments Held	1		1
Including accrued interest [Member] | Excluding netting [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	29,100,000		29,100,000		37,100,000
Financial derivatives, at fair value	99,500,000		99,500,000		168,000,000
Including accrued interest [Member] | Netting [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	3,800,000		3,800,000		2,400,000
Non-Interest Income [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	24,000	(1,678,000)	14,043,000	(26,570,000)
Derivative Contract Type - Centrally Cleared [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative, Notional Amount	1,200,000,000		1,200,000,000
Interest Rate Swap, Pay Fixed Non-Callable [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount of Interest Rate Fair Value Hedge Derivatives	900,000,000		900,000,000		950,000,000
Interest Rate Swap, Pay Fixed Non-Callable [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative, Notional Amount	784,349,000		784,349,000		805,622,000
Financial derivatives, at fair value	55,157,000		55,157,000		91,205,000
Interest Rate Swap, Pay Fixed Non-Callable [Member] | Dedesignated hedge [Member]
Derivative [Line Items]
Notional Amount of Interest Rate Fair Value Hedge Derivatives	50,000,000		50,000,000
Counterparty level [Member] | Including accrued interest [Member] | Netting [Member]
Derivative [Line Items]
Financial derivatives, at fair value	76,000,000		76,000,000		135,800,000
Basis Swap [Member] | Zions First National Bank [Member]
Derivative [Line Items]
Derivative, Notional Amount	29,300,000		29,300,000		49,300,000
Derivative, Fair Value, Net	(300,000)		(300,000)		(700,000)
Gain (Loss) on Derivative Instruments, Net, Pretax	100,000	100,000	400,000	500,000
Basis Swap [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative, Notional Amount	374,288,000		374,288,000		609,262,000
Financial derivatives, at fair value	$ 358,000		$ 358,000		$ 784,000