Derivatives, Fair Value of Derivatives Contracts (Details) (USD $)	Sep. 30, 2014		Dec. 31, 2013		Sep. 30, 2013
Derivative Assets, Fair Value, Net [Abstract]
Derviative Assets, Fair Value, Net of Cash Collateral, assets	$ 360,809,000		$ 265,012,000		$ 377,325,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Fair Value, Net of Cash Collateral	348,687,000		247,185,000		232,544,000
Increase in cash margin obligation for derivative holdings with a decrease in credit rating from A1 to below investment grade	19,000,000
Not Designated as Hedging Instrument [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	15,733,499,000	[1]	13,812,049,000	[1]	15,869,618,000	[1]
Derivative Liabilities, Notional	16,355,034,000	[1]	13,974,098,000	[1]	15,977,434,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	406,454,000		351,683,000		531,960,000
Derivative Assets, Netting Adjustments	(41,393,000)		(77,746,000)		(145,478,000)
Derivative Assets, Net Fair Value Before Cash Collateral	365,061,000		273,937,000		386,482,000
Derivative Assets, Cash Collateral	(4,252,000)		(8,925,000)		(9,157,000)
Derviative Assets, Fair Value, Net of Cash Collateral, assets	360,809,000		265,012,000		377,325,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	408,842,000		349,133,000		530,432,000
Derivative Liabilities, Netting Adjustments	(41,393,000)		(77,746,000)		(145,478,000)
Derivative Liabilities, Net Fair Value Before Cash Collateral	367,449,000		271,387,000		384,954,000
Derivative Liabilities, Cash Collateral	(18,762,000)		(24,202,000)		(152,410,000)
Derivative Liabilities, Fair Value, Net of Cash Collateral	348,687,000		247,185,000		232,544,000
Not Designated as Hedging Instrument [Member] | To-be-announced residential mortgage-backed securities [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	13,125,309,000	[1]	10,817,159,000	[1]	12,455,689,000	[1]
Derivative Liabilities, Notional	13,702,440,000	[1]	10,982,049,000	[1]	12,529,704,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	48,913,000		102,921,000		224,392,000
Derivative Assets, Netting Adjustments	(25,263,000)		(46,623,000)		(99,970,000)
Derivative Assets, Net Fair Value Before Cash Collateral	23,650,000		56,298,000		124,422,000
Derivative Assets, Cash Collateral	0		0		(5,191,000)
Derviative Assets, Fair Value, Net of Cash Collateral, assets	23,650,000		56,298,000		119,231,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	45,889,000		99,830,000		221,720,000
Derivative Liabilities, Netting Adjustments	(25,263,000)		(46,623,000)		(99,970,000)
Derivative Liabilities, Net Fair Value Before Cash Collateral	20,626,000		53,207,000		121,750,000
Derivative Liabilities, Cash Collateral	0		0		(118,166,000)
Derivative Liabilities, Fair Value, Net of Cash Collateral	20,626,000		53,207,000		3,584,000
Not Designated as Hedging Instrument [Member] | Interest rate swaps [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	1,171,163,000	[1]	1,283,379,000	[1]	1,361,499,000	[1]
Derivative Liabilities, Notional	1,171,163,000	[1]	1,283,379,000	[1]	1,361,499,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	34,148,000		44,124,000		49,183,000
Derivative Assets, Netting Adjustments	0		0		0
Derivative Assets, Net Fair Value Before Cash Collateral	34,148,000		44,124,000		49,183,000
Derivative Assets, Cash Collateral	(199,000)		(731,000)		0
Derviative Assets, Fair Value, Net of Cash Collateral, assets	33,949,000		43,393,000		49,183,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	34,316,000		44,377,000		49,518,000
Derivative Liabilities, Netting Adjustments	0		0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral	34,316,000		44,377,000		49,518,000
Derivative Liabilities, Cash Collateral	(15,145,000)		(17,853,000)		(21,240,000)
Derivative Liabilities, Fair Value, Net of Cash Collateral	19,171,000		26,524,000		28,278,000
Not Designated as Hedging Instrument [Member] | Energy contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	847,446,000	[1]	1,263,266,000	[1]	1,412,238,000	[1]
Derivative Liabilities, Notional	844,976,000	[1]	1,216,426,000	[1]	1,400,542,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	32,005,000		48,078,000		73,293,000
Derivative Assets, Netting Adjustments	(15,660,000)		(29,957,000)		(42,078,000)
Derivative Assets, Net Fair Value Before Cash Collateral	16,345,000		18,121,000		31,215,000
Derivative Assets, Cash Collateral	(3,499,000)		(2,575,000)		(606,000)
Derviative Assets, Fair Value, Net of Cash Collateral, assets	12,846,000		15,546,000		30,609,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	35,583,000		46,095,000		71,971,000
Derivative Liabilities, Netting Adjustments	(15,660,000)		(29,957,000)		(42,078,000)
Derivative Liabilities, Net Fair Value Before Cash Collateral	19,923,000		16,138,000		29,893,000
Derivative Liabilities, Cash Collateral	0		(6,055,000)		(10,762,000)
Derivative Liabilities, Fair Value, Net of Cash Collateral	19,923,000		10,083,000		19,131,000
Not Designated as Hedging Instrument [Member] | Agricultural contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	49,943,000	[1]	100,886,000	[1]	262,770,000	[1]
Derivative Liabilities, Notional	49,911,000	[1]	99,191,000	[1]	261,782,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	2,372,000		2,060,000		5,783,000
Derivative Assets, Netting Adjustments	(470,000)		(1,166,000)		(3,430,000)
Derivative Assets, Net Fair Value Before Cash Collateral	1,902,000		894,000		2,353,000
Derivative Assets, Cash Collateral	0		0		0
Derviative Assets, Fair Value, Net of Cash Collateral, assets	1,902,000		894,000		2,353,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	2,404,000		2,009,000		5,731,000
Derivative Liabilities, Netting Adjustments	(470,000)		(1,166,000)		(3,430,000)
Derivative Liabilities, Net Fair Value Before Cash Collateral	1,934,000		843,000		2,301,000
Derivative Liabilities, Cash Collateral	(1,888,000)		0		(2,242,000)
Derivative Liabilities, Fair Value, Net of Cash Collateral	46,000		843,000		59,000
Not Designated as Hedging Instrument [Member] | Foreign exchange contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	336,755,000	[1]	136,543,000	[1]	164,970,000	[1]
Derivative Liabilities, Notional	336,661,000	[1]	135,237,000	[1]	164,455,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	275,116,000		136,543,000		164,970,000
Derivative Assets, Netting Adjustments	0		0		0
Derivative Assets, Net Fair Value Before Cash Collateral	275,116,000		136,543,000		164,970,000
Derivative Assets, Cash Collateral	0		(2,147,000)		0
Derviative Assets, Fair Value, Net of Cash Collateral, assets	275,116,000		134,396,000		164,970,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	274,829,000		135,237,000		164,455,000
Derivative Liabilities, Netting Adjustments	0		0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral	274,829,000		135,237,000		164,455,000
Derivative Liabilities, Cash Collateral	(1,729,000)		(294,000)		0
Derivative Liabilities, Fair Value, Net of Cash Collateral	273,100,000		134,943,000		164,455,000
Not Designated as Hedging Instrument [Member] | Equity option contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	202,883,000	[1]	210,816,000	[1]	212,452,000	[1]
Derivative Liabilities, Notional	202,883,000	[1]	210,816,000	[1]	212,452,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	13,900,000		17,957,000		14,339,000
Derivative Assets, Netting Adjustments	0		0		0
Derivative Assets, Net Fair Value Before Cash Collateral	13,900,000		17,957,000		14,339,000
Derivative Assets, Cash Collateral	(554,000)		(3,472,000)		(3,360,000)
Derviative Assets, Fair Value, Net of Cash Collateral, assets	13,346,000		14,485,000		10,979,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	13,900,000		17,957,000		14,339,000
Derivative Liabilities, Netting Adjustments	0		0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral	13,900,000		17,957,000		14,339,000
Derivative Liabilities, Cash Collateral	0		0		0
Derivative Liabilities, Fair Value, Net of Cash Collateral	13,900,000		17,957,000		14,339,000
Not Designated as Hedging Instrument [Member] | Total customer risk management programs [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	15,733,499,000	[1]	13,812,049,000	[1]	15,869,618,000	[1]
Derivative Liabilities, Notional	16,308,034,000	[1]	13,927,098,000	[1]	15,930,434,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	406,454,000		351,683,000		531,960,000
Derivative Assets, Netting Adjustments	(41,393,000)		(77,746,000)		(145,478,000)
Derivative Assets, Net Fair Value Before Cash Collateral	365,061,000		273,937,000		386,482,000
Derivative Assets, Cash Collateral	(4,252,000)		(8,925,000)		(9,157,000)
Derviative Assets, Fair Value, Net of Cash Collateral, assets	360,809,000		265,012,000		377,325,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	406,921,000		345,505,000		527,734,000
Derivative Liabilities, Netting Adjustments	(41,393,000)		(77,746,000)		(145,478,000)
Derivative Liabilities, Net Fair Value Before Cash Collateral	365,528,000		267,759,000		382,256,000
Derivative Liabilities, Cash Collateral	(18,762,000)		(24,202,000)		(152,410,000)
Derivative Liabilities, Fair Value, Net of Cash Collateral	346,766,000		243,557,000		229,846,000
Not Designated as Hedging Instrument [Member] | Interest rate risk management programs [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	0	[1]	0	[1]	0	[1]
Derivative Liabilities, Notional	47,000,000	[1]	47,000,000	[1]	47,000,000	[1]
Derivatives, Notional	47,000,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	0		0		0
Derivative Assets, Netting Adjustments	0		0		0
Derivative Assets, Net Fair Value Before Cash Collateral	0		0		0
Derivative Assets, Cash Collateral	0		0		0
Derviative Assets, Fair Value, Net of Cash Collateral, assets	0		0		0
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	1,921,000		3,628,000		2,698,000
Derivative Liabilities, Netting Adjustments	0		0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral	1,921,000		3,628,000		2,698,000
Derivative Liabilities, Cash Collateral	0		0		0
Derivative Liabilities, Fair Value, Net of Cash Collateral	$ 1,921,000		$ 3,628,000		$ 2,698,000

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