Derivatives, Fair Value of Derivatives Contracts (Details) (USD $)	Jun. 30, 2014		Dec. 31, 2013		Jun. 30, 2013
Derivative Assets, Fair Value, Net [Abstract]
Derviative contracts, net of cash collateral, assets	$ 357,680,000		$ 265,012,000		$ 546,206,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Fair Value, Net of Cash Collateral	297,851,000		247,185,000		521,991,000
Increase in cash margin obligation for derivative holdings with a decrease in credit rating from A1 to below investment grade	21,000,000
Not Designated as Hedging Instrument [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	17,391,369,000	[1]	13,812,049,000	[1]	19,831,867,000	[1]
Derivative Liabilities, Notional	17,581,900,000	[1]	13,974,098,000	[1]	19,906,435,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	436,508,000		351,683,000		859,432,000
Derivative Assets, Netting Adjustments	(77,138,000)		(77,746,000)		(308,121,000)
Derivative Assets, Net Fair Value Before Cash Collateral	359,370,000		273,937,000		551,311,000
Derivative Asset, Cash Collateral	(1,690,000)		(8,925,000)		(5,105,000)
Derviative contracts, net of cash collateral, assets	357,680,000		265,012,000		546,206,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	434,022,000		349,133,000		855,358,000
Derivative Liabilities, Netting Adjustments	(77,138,000)		(77,746,000)		(308,121,000)
Derivative Liabilities, Net Fair Value Before Cash Collateral	356,884,000		271,387,000		547,237,000
Derivative Liabilities, Fair Value, Cash Collateral	(59,033,000)		(24,202,000)		(25,246,000)
Derivative Liabilities, Fair Value, Net of Cash Collateral	297,851,000		247,185,000		521,991,000
Not Designated as Hedging Instrument [Member] | To-be-announced residential mortgage-backed securities [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	14,576,481,000	[1]	10,817,159,000	[1]	16,351,395,000	[1]
Derivative Liabilities, Notional	14,734,106,000	[1]	10,982,049,000	[1]	16,439,531,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	134,411,000		102,921,000		545,290,000
Derivative Assets, Netting Adjustments	(53,746,000)		(46,623,000)		(268,087,000)
Derivative Assets, Net Fair Value Before Cash Collateral	80,665,000		56,298,000		277,203,000
Derivative Asset, Cash Collateral	0		0		0
Derviative contracts, net of cash collateral, assets	80,665,000		56,298,000		277,203,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	131,256,000		99,830,000		540,540,000
Derivative Liabilities, Netting Adjustments	(53,746,000)		(46,623,000)		(268,087,000)
Derivative Liabilities, Net Fair Value Before Cash Collateral	77,510,000		53,207,000		272,453,000
Derivative Liabilities, Fair Value, Cash Collateral	0		0		0
Derivative Liabilities, Fair Value, Net of Cash Collateral	77,510,000		53,207,000		272,453,000
Not Designated as Hedging Instrument [Member] | Interest rate swaps [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	1,266,228,000	[1]	1,283,379,000	[1]	1,381,836,000	[1]
Derivative Liabilities, Notional	1,266,228,000	[1]	1,283,379,000	[1]	1,381,836,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	39,974,000		44,124,000		51,745,000
Derivative Assets, Netting Adjustments	0		0		0
Derivative Assets, Net Fair Value Before Cash Collateral	39,974,000		44,124,000		51,745,000
Derivative Asset, Cash Collateral	0		(731,000)		0
Derviative contracts, net of cash collateral, assets	39,974,000		43,393,000		51,745,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	40,218,000		44,377,000		52,095,000
Derivative Liabilities, Netting Adjustments	0		0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral	40,218,000		44,377,000		52,095,000
Derivative Liabilities, Fair Value, Cash Collateral	(19,700,000)		(17,853,000)		(19,381,000)
Derivative Liabilities, Fair Value, Net of Cash Collateral	20,518,000		26,524,000		32,714,000
Not Designated as Hedging Instrument [Member] | Energy contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	1,063,840,000	[1]	1,263,266,000	[1]	1,501,959,000	[1]
Derivative Liabilities, Notional	1,049,835,000	[1]	1,216,426,000	[1]	1,441,957,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	67,831,000		48,078,000		65,414,000
Derivative Assets, Netting Adjustments	(23,169,000)		(29,957,000)		(35,376,000)
Derivative Assets, Net Fair Value Before Cash Collateral	44,662,000		18,121,000		30,038,000
Derivative Asset, Cash Collateral	0		(2,575,000)		(2,537,000)
Derviative contracts, net of cash collateral, assets	44,662,000		15,546,000		27,501,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	66,742,000		46,095,000		63,515,000
Derivative Liabilities, Netting Adjustments	(23,169,000)		(29,957,000)		(35,376,000)
Derivative Liabilities, Net Fair Value Before Cash Collateral	43,573,000		16,138,000		28,139,000
Derivative Liabilities, Fair Value, Cash Collateral	(36,355,000)		(6,055,000)		(5,865,000)
Derivative Liabilities, Fair Value, Net of Cash Collateral	7,218,000		10,083,000		22,274,000
Not Designated as Hedging Instrument [Member] | Agricultural contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	36,050,000	[1]	100,886,000	[1]	207,439,000	[1]
Derivative Liabilities, Notional	36,036,000	[1]	99,191,000	[1]	207,329,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	2,528,000		2,060,000		5,871,000
Derivative Assets, Netting Adjustments	(223,000)		(1,166,000)		(4,658,000)
Derivative Assets, Net Fair Value Before Cash Collateral	2,305,000		894,000		1,213,000
Derivative Asset, Cash Collateral	0		0		0
Derviative contracts, net of cash collateral, assets	2,305,000		894,000		1,213,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	2,538,000		2,009,000		5,824,000
Derivative Liabilities, Netting Adjustments	(223,000)		(1,166,000)		(4,658,000)
Derivative Liabilities, Net Fair Value Before Cash Collateral	2,315,000		843,000		1,166,000
Derivative Liabilities, Fair Value, Cash Collateral	(2,298,000)		0		0
Derivative Liabilities, Fair Value, Net of Cash Collateral	17,000		843,000		1,166,000
Not Designated as Hedging Instrument [Member] | Foreign exchange contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	242,866,000	[1]	136,543,000	[1]	177,643,000	[1]
Derivative Liabilities, Notional	242,791,000	[1]	135,237,000	[1]	177,187,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	174,802,000		136,543,000		177,643,000
Derivative Assets, Netting Adjustments	0		0		0
Derivative Assets, Net Fair Value Before Cash Collateral	174,802,000		136,543,000		177,643,000
Derivative Asset, Cash Collateral	0		(2,147,000)		0
Derviative contracts, net of cash collateral, assets	174,802,000		134,396,000		177,643,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	174,477,000		135,237,000		177,187,000
Derivative Liabilities, Netting Adjustments	0		0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral	174,477,000		135,237,000		177,187,000
Derivative Liabilities, Fair Value, Cash Collateral	(680,000)		(294,000)		0
Derivative Liabilities, Fair Value, Net of Cash Collateral	173,797,000		134,943,000		177,187,000
Not Designated as Hedging Instrument [Member] | Equity option contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	205,904,000	[1]	210,816,000	[1]	211,595,000	[1]
Derivative Liabilities, Notional	205,904,000	[1]	210,816,000	[1]	211,595,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	16,962,000		17,957,000		13,469,000
Derivative Assets, Netting Adjustments	0		0		0
Derivative Assets, Net Fair Value Before Cash Collateral	16,962,000		17,957,000		13,469,000
Derivative Asset, Cash Collateral	(1,690,000)		(3,472,000)		(2,568,000)
Derviative contracts, net of cash collateral, assets	15,272,000		14,485,000		10,901,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	16,962,000		17,957,000		13,469,000
Derivative Liabilities, Netting Adjustments	0		0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral	16,962,000		17,957,000		13,469,000
Derivative Liabilities, Fair Value, Cash Collateral	0		0		0
Derivative Liabilities, Fair Value, Net of Cash Collateral	16,962,000		17,957,000		13,469,000
Not Designated as Hedging Instrument [Member] | Customer risk management programs [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	17,391,369,000	[1]	13,812,049,000	[1]	19,831,867,000	[1]
Derivative Liabilities, Notional	17,534,900,000	[1]	13,927,098,000	[1]	19,859,435,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	436,508,000		351,683,000		859,432,000
Derivative Assets, Netting Adjustments	(77,138,000)		(77,746,000)		(308,121,000)
Derivative Assets, Net Fair Value Before Cash Collateral	359,370,000		273,937,000		551,311,000
Derivative Asset, Cash Collateral	(1,690,000)		(8,925,000)		(5,105,000)
Derviative contracts, net of cash collateral, assets	357,680,000		265,012,000		546,206,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	432,193,000		345,505,000		852,630,000
Derivative Liabilities, Netting Adjustments	(77,138,000)		(77,746,000)		(308,121,000)
Derivative Liabilities, Net Fair Value Before Cash Collateral	355,055,000		267,759,000		544,509,000
Derivative Liabilities, Fair Value, Cash Collateral	(59,033,000)		(24,202,000)		(25,246,000)
Derivative Liabilities, Fair Value, Net of Cash Collateral	296,022,000		243,557,000		519,263,000
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 Asset, Cash Collateral	0		0		0
Derviative contracts, net of cash collateral, assets	0		0		0
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	1,829,000		3,628,000		2,728,000
Derivative Liabilities, Netting Adjustments	0		0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral	1,829,000		3,628,000		2,728,000
Derivative Liabilities, Fair Value, Cash Collateral	0		0		0
Derivative Liabilities, Fair Value, Net of Cash Collateral	$ 1,829,000		$ 3,628,000		$ 2,728,000

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