Derivatives, Fair Value of Derivatives Contracts (Details) (USD $)	Sep. 30, 2013		Dec. 31, 2012		Sep. 30, 2012
Derivative Asset, Fair Value, Net [Abstract]
Derviative contracts, net of cash margin, assets	$ 377,325,000		$ 338,106,000		$ 435,653,000
Derivative Liability, Fair Value, Net [Abstract]
Derivative Liability, Fair Value, Net of Cash Collateral	232,544,000		283,589,000		254,422,000
Increase in cash margin obligation for derivative holdings with a decrease in credit rating from A1 to below investment grade	27,000,000
Not Designated as Hedging Instrument [Member]
Notional Amount of Derivatives [Abstract]
Derivative Asset, Notional	15,869,618,000	[1]	16,188,105,000	[1]	18,348,043,000	[1]
Derivative Liability, Notional	15,977,434,000	[1]	16,547,182,000	[1]	18,223,460,000	[1]
Derivative Asset, Fair Value, Net [Abstract]
Derivative Asset, Gross Fair Value	531,960,000		404,875,000		641,420,000
Derivative Asset, Netting Adjustments	(145,478,000)		(63,305,000)		(194,614,000)
Derivative Assets, Net Fair Value Before Cash Collateral	386,482,000		341,570,000		446,806,000
Derivative Asset, Cash Collateral	(9,157,000)		(3,464,000)		(11,153,000)
Derviative contracts, net of cash margin, assets	377,325,000		338,106,000		435,653,000
Derivative Liability, Fair Value, Net [Abstract]
Derivative Liability, Gross Fair Value	530,432,000		396,263,000		633,658,000
Derivative Liability, Netting Adjustments	(145,478,000)		(63,305,000)		(194,614,000)
Derivative Liability, Net Fair Value Before Cash Collateral	384,954,000		332,958,000		439,044,000
Derivative Liability, Fair Value, Cash Collateral	(152,410,000)		(49,369,000)		(184,622,000)
Derivative Liability, Fair Value, Net of Cash Collateral	232,544,000		283,589,000		254,422,000
Not Designated as Hedging Instrument [Member] | To-be-announced residential mortgage-backed securities [Member]
Notional Amount of Derivatives [Abstract]
Derivative Asset, Notional	12,455,689,000	[1]	12,850,805,000	[1]	14,858,520,000	[1]
Derivative Liability, Notional	12,529,704,000	[1]	13,239,078,000	[1]	14,738,232,000	[1]
Derivative Asset, Fair Value, Net [Abstract]
Derivative Asset, Gross Fair Value	224,392,000		46,113,000		276,678,000
Derivative Asset, Netting Adjustments	(99,970,000)		(15,656,000)		(121,573,000)
Derivative Assets, Net Fair Value Before Cash Collateral	124,422,000		30,457,000		155,105,000
Derivative Asset, Cash Collateral	(5,191,000)		0		(7,287,000)
Derviative contracts, net of cash margin, assets	119,231,000		30,457,000		147,818,000
Derivative Liability, Fair Value, Net [Abstract]
Derivative Liability, Gross Fair Value	221,720,000		43,064,000		274,195,000
Derivative Liability, Netting Adjustments	(99,970,000)		(15,656,000)		(121,573,000)
Derivative Liability, Net Fair Value Before Cash Collateral	121,750,000		27,408,000		152,622,000
Derivative Liability, Fair Value, Cash Collateral	(118,166,000)		(15,467,000)		(143,945,000)
Derivative Liability, Fair Value, Net of Cash Collateral	3,584,000		11,941,000		8,677,000
Not Designated as Hedging Instrument [Member] | Interest rate swaps [Member]
Notional Amount of Derivatives [Abstract]
Derivative Asset, Notional	1,361,499,000	[1]	1,319,827,000	[1]	1,301,109,000	[1]
Derivative Liability, Notional	1,361,499,000	[1]	1,319,827,000	[1]	1,301,109,000	[1]
Derivative Asset, Fair Value, Net [Abstract]
Derivative Asset, Gross Fair Value	49,183,000		72,201,000		79,350,000
Derivative Asset, Netting Adjustments	0		0		0
Derivative Assets, Net Fair Value Before Cash Collateral	49,183,000		72,201,000		79,350,000
Derivative Asset, Cash Collateral	0		0		0
Derviative contracts, net of cash margin, assets	49,183,000		72,201,000		79,350,000
Derivative Liability, Fair Value, Net [Abstract]
Derivative Liability, Gross Fair Value	49,518,000		72,724,000		79,937,000
Derivative Liability, Netting Adjustments	0		0		0
Derivative Liability, Net Fair Value Before Cash Collateral	49,518,000		72,724,000		79,937,000
Derivative Liability, Fair Value, Cash Collateral	(21,240,000)		(31,945,000)		(37,841,000)
Derivative Liability, Fair Value, Net of Cash Collateral	28,278,000		40,779,000		42,096,000
Not Designated as Hedging Instrument [Member] | Energy contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Asset, Notional	1,412,238,000	[1]	1,346,780,000	[1]	1,556,164,000	[1]
Derivative Liability, Notional	1,400,542,000	[1]	1,334,349,000	[1]	1,596,791,000	[1]
Derivative Asset, Fair Value, Net [Abstract]
Derivative Asset, Gross Fair Value	73,293,000		82,349,000		105,588,000
Derivative Asset, Netting Adjustments	(42,078,000)		(44,485,000)		(67,030,000)
Derivative Assets, Net Fair Value Before Cash Collateral	31,215,000		37,864,000		38,558,000
Derivative Asset, Cash Collateral	(606,000)		(3,464,000)		(3,866,000)
Derviative contracts, net of cash margin, assets	30,609,000		34,400,000		34,692,000
Derivative Liability, Fair Value, Net [Abstract]
Derivative Liability, Gross Fair Value	71,971,000		83,654,000		107,556,000
Derivative Liability, Netting Adjustments	(42,078,000)		(44,485,000)		(67,030,000)
Derivative Liability, Net Fair Value Before Cash Collateral	29,893,000		39,169,000		40,526,000
Derivative Liability, Fair Value, Cash Collateral	(10,762,000)		(1,769,000)		(2,836,000)
Derivative Liability, Fair Value, Net of Cash Collateral	19,131,000		37,400,000		37,690,000
Not Designated as Hedging Instrument [Member] | Agricultural contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Asset, Notional	262,770,000	[1]	212,434,000	[1]	198,735,000	[1]
Derivative Liability, Notional	261,782,000	[1]	212,135,000	[1]	195,068,000	[1]
Derivative Asset, Fair Value, Net [Abstract]
Derivative Asset, Gross Fair Value	5,783,000		3,638,000		6,835,000
Derivative Asset, Netting Adjustments	(3,430,000)		(3,164,000)		(6,011,000)
Derivative Assets, Net Fair Value Before Cash Collateral	2,353,000		474,000		824,000
Derivative Asset, Cash Collateral	0		0		0
Derviative contracts, net of cash margin, assets	2,353,000		474,000		824,000
Derivative Liability, Fair Value, Net [Abstract]
Derivative Liability, Gross Fair Value	5,731,000		3,571,000		6,750,000
Derivative Liability, Netting Adjustments	(3,430,000)		(3,164,000)		(6,011,000)
Derivative Liability, Net Fair Value Before Cash Collateral	2,301,000		407,000		739,000
Derivative Liability, Fair Value, Cash Collateral	(2,242,000)		(188,000)		0
Derivative Liability, Fair Value, Net of Cash Collateral	59,000		219,000		739,000
Not Designated as Hedging Instrument [Member] | Foreign exchange contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Asset, Notional	164,970,000	[1]	180,318,000	[1]	150,232,000	[1]
Derivative Liability, Notional	164,455,000	[1]	179,852,000	[1]	149,977,000	[1]
Derivative Asset, Fair Value, Net [Abstract]
Derivative Asset, Gross Fair Value	164,970,000		180,318,000		150,232,000
Derivative Asset, Netting Adjustments	0		0		0
Derivative Assets, Net Fair Value Before Cash Collateral	164,970,000		180,318,000		150,232,000
Derivative Asset, Cash Collateral	0		0		0
Derviative contracts, net of cash margin, assets	164,970,000		180,318,000		150,232,000
Derivative Liability, Fair Value, Net [Abstract]
Derivative Liability, Gross Fair Value	164,455,000		179,852,000		149,977,000
Derivative Liability, Netting Adjustments	0		0		0
Derivative Liability, Net Fair Value Before Cash Collateral	164,455,000		179,852,000		149,977,000
Derivative Liability, Fair Value, Cash Collateral	0		0		0
Derivative Liability, Fair Value, Net of Cash Collateral	164,455,000		179,852,000		149,977,000
Not Designated as Hedging Instrument [Member] | Equity option contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Asset, Notional	212,452,000	[1]	211,941,000	[1]	217,283,000	[1]
Derivative Liability, Notional	212,452,000	[1]	211,941,000	[1]	217,283,000	[1]
Derivative Asset, Fair Value, Net [Abstract]
Derivative Asset, Gross Fair Value	14,339,000		12,593,000		14,460,000
Derivative Asset, Netting Adjustments	0		0		0
Derivative Assets, Net Fair Value Before Cash Collateral	14,339,000		12,593,000		14,460,000
Derivative Asset, Cash Collateral	(3,360,000)		0		0
Derviative contracts, net of cash margin, assets	10,979,000		12,593,000		14,460,000
Derivative Liability, Fair Value, Net [Abstract]
Derivative Liability, Gross Fair Value	14,339,000		12,593,000		14,460,000
Derivative Liability, Netting Adjustments	0		0		0
Derivative Liability, Net Fair Value Before Cash Collateral	14,339,000		12,593,000		14,460,000
Derivative Liability, Fair Value, Cash Collateral	0		0		0
Derivative Liability, Fair Value, Net of Cash Collateral	14,339,000		12,593,000		14,460,000
Not Designated as Hedging Instrument [Member] | Customer risk management programs [Member]
Notional Amount of Derivatives [Abstract]
Derivative Asset, Notional	15,869,618,000	[1]	16,122,105,000	[1]	18,282,043,000	[1]
Derivative Liability, Notional	15,930,434,000	[1]	16,497,182,000	[1]	18,198,460,000	[1]
Derivative Asset, Fair Value, Net [Abstract]
Derivative Asset, Gross Fair Value	531,960,000		397,212,000		633,143,000
Derivative Asset, Netting Adjustments	(145,478,000)		(63,305,000)		(194,614,000)
Derivative Assets, Net Fair Value Before Cash Collateral	386,482,000		333,907,000		438,529,000
Derivative Asset, Cash Collateral	(9,157,000)		(3,464,000)		(11,153,000)
Derviative contracts, net of cash margin, assets	377,325,000		330,443,000		427,376,000
Derivative Liability, Fair Value, Net [Abstract]
Derivative Liability, Gross Fair Value	527,734,000		395,458,000		632,875,000
Derivative Liability, Netting Adjustments	(145,478,000)		(63,305,000)		(194,614,000)
Derivative Liability, Net Fair Value Before Cash Collateral	382,256,000		332,153,000		438,261,000
Derivative Liability, Fair Value, Cash Collateral	(152,410,000)		(49,369,000)		(184,622,000)
Derivative Liability, Fair Value, Net of Cash Collateral	229,846,000		282,784,000		253,639,000
Not Designated as Hedging Instrument [Member] | Interest rate risk management programs [Member]
Notional Amount of Derivatives [Abstract]
Derivative Asset, Notional	0	[1]	66,000,000	[1]	66,000,000	[1]
Derivative Liability, Notional	47,000,000	[1]	50,000,000	[1]	25,000,000	[1]
Derivatives, Notional	47,000,000	[1]
Derivative Asset, Fair Value, Net [Abstract]
Derivative Asset, Gross Fair Value	0		7,663,000		8,277,000
Derivative Asset, Netting Adjustments	0		0		0
Derivative Assets, Net Fair Value Before Cash Collateral	0		7,663,000		8,277,000
Derivative Asset, Cash Collateral	0		0		0
Derviative contracts, net of cash margin, assets	0		7,663,000		8,277,000
Derivative Liability, Fair Value, Net [Abstract]
Derivative Liability, Gross Fair Value	2,698,000		805,000		783,000
Derivative Liability, Netting Adjustments	0		0		0
Derivative Liability, Net Fair Value Before Cash Collateral	2,698,000		805,000		783,000
Derivative Liability, Fair Value, Cash Collateral	0		0		0
Derivative Liability, Fair Value, Net of Cash Collateral	$ 2,698,000		$ 805,000		$ 783,000

[1] Notional amounts for commodity contracts are converted into dollar-equivalent amounts based on dollar prices at the inception of the contract.