Derivatives, Fair Value of Derivatives Contracts (Details) (USD $)	Mar. 31, 2014		Dec. 31, 2013		Mar. 31, 2013
Derivative Assets, Fair Value, Net [Abstract]
Derviative contracts, net of cash margin, assets	$ 218,507,000		$ 265,012,000		$ 320,473,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Fair Value, Net of Cash Collateral	185,499,000		247,185,000		251,836,000
Increase in cash margin obligation for derivative holdings with a decrease in credit rating from A1 to below investment grade	26,000,000
Not Designated as Hedging Instrument [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	13,778,569,000	[1]	13,812,049,000	[1]	15,802,857,000	[1]
Derivative Liabilities, Notional	14,283,964,000	[1]	13,974,098,000	[1]	16,177,283,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	271,093,000		351,683,000		382,706,000
Derivative Assets, Netting Adjustments	(49,206,000)		(77,746,000)		(60,611,000)
Derivative Assets, Net Fair Value Before Cash Collateral	221,887,000		273,937,000		322,095,000
Derivative Asset, Cash Collateral	(3,380,000)		(8,925,000)		(1,622,000)
Derviative contracts, net of cash margin, assets	218,507,000		265,012,000		320,473,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	268,582,000		349,133,000		379,664,000
Derivative Liabilities, Netting Adjustments	(49,206,000)		(77,746,000)		(60,611,000)
Derivative Liabilities, Net Fair Value Before Cash Collateral	219,376,000		271,387,000		319,053,000
Derivative Liabilities, Fair Value, Cash Collateral	(33,877,000)		(24,202,000)		(67,217,000)
Derivative Liabilities, Fair Value, Net of Cash Collateral	185,499,000		247,185,000		251,836,000
Not Designated as Hedging Instrument [Member] | To-be-announced residential mortgage-backed securities [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	10,859,613,000	[1]	10,817,159,000	[1]	12,428,736,000	[1]
Derivative Liabilities, Notional	11,398,442,000	[1]	10,982,049,000	[1]	12,827,390,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	30,897,000		102,921,000		59,599,000
Derivative Assets, Netting Adjustments	(20,219,000)		(46,623,000)		(21,727,000)
Derivative Assets, Net Fair Value Before Cash Collateral	10,678,000		56,298,000		37,872,000
Derivative Asset, Cash Collateral	0		0		0
Derviative contracts, net of cash margin, assets	10,678,000		56,298,000		37,872,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	27,966,000		99,830,000		56,565,000
Derivative Liabilities, Netting Adjustments	(20,219,000)		(46,623,000)		(21,727,000)
Derivative Liabilities, Net Fair Value Before Cash Collateral	7,747,000		53,207,000		34,838,000
Derivative Liabilities, Fair Value, Cash Collateral	0		0		(21,657,000)
Derivative Liabilities, Fair Value, Net of Cash Collateral	7,747,000		53,207,000		13,181,000
Not Designated as Hedging Instrument [Member] | Interest rate swaps [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	1,266,880,000	[1]	1,283,379,000	[1]	1,380,439,000	[1]
Derivative Liabilities, Notional	1,266,880,000	[1]	1,283,379,000	[1]	1,380,439,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	41,331,000		44,124,000		65,654,000
Derivative Assets, Netting Adjustments	0		0		0
Derivative Assets, Net Fair Value Before Cash Collateral	41,331,000		44,124,000		65,654,000
Derivative Asset, Cash Collateral	0		(731,000)		0
Derviative contracts, net of cash margin, assets	41,331,000		43,393,000		65,654,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	41,596,000		44,377,000		66,149,000
Derivative Liabilities, Netting Adjustments	0		0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral	41,596,000		44,377,000		66,149,000
Derivative Liabilities, Fair Value, Cash Collateral	(17,388,000)		(17,853,000)		(35,127,000)
Derivative Liabilities, Fair Value, Net of Cash Collateral	24,208,000		26,524,000		31,022,000
Not Designated as Hedging Instrument [Member] | Energy contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	1,207,861,000	[1]	1,263,266,000	[1]	1,415,266,000	[1]
Derivative Liabilities, Notional	1,134,208,000	[1]	1,216,426,000	[1]	1,388,495,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	53,440,000		48,078,000		62,426,000
Derivative Assets, Netting Adjustments	(27,112,000)		(29,957,000)		(35,440,000)
Derivative Assets, Net Fair Value Before Cash Collateral	26,328,000		18,121,000		26,986,000
Derivative Asset, Cash Collateral	0		(2,575,000)		(1,622,000)
Derviative contracts, net of cash margin, assets	26,328,000		15,546,000		25,364,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	51,308,000		46,095,000		62,185,000
Derivative Liabilities, Netting Adjustments	(27,112,000)		(29,957,000)		(35,440,000)
Derivative Liabilities, Net Fair Value Before Cash Collateral	24,196,000		16,138,000		26,745,000
Derivative Liabilities, Fair Value, Cash Collateral	(14,202,000)		(6,055,000)		(10,433,000)
Derivative Liabilities, Fair Value, Net of Cash Collateral	9,994,000		10,083,000		16,312,000
Not Designated as Hedging Instrument [Member] | Agricultural contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	111,960,000	[1]	100,886,000	[1]	167,652,000	[1]
Derivative Liabilities, Notional	105,518,000	[1]	99,191,000	[1]	167,642,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	4,208,000		2,060,000		4,174,000
Derivative Assets, Netting Adjustments	(1,875,000)		(1,166,000)		(3,444,000)
Derivative Assets, Net Fair Value Before Cash Collateral	2,333,000		894,000		730,000
Derivative Asset, Cash Collateral	0		0		0
Derviative contracts, net of cash margin, assets	2,333,000		894,000		730,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	4,174,000		2,009,000		4,157,000
Derivative Liabilities, Netting Adjustments	(1,875,000)		(1,166,000)		(3,444,000)
Derivative Liabilities, Net Fair Value Before Cash Collateral	2,299,000		843,000		713,000
Derivative Liabilities, Fair Value, Cash Collateral	(2,287,000)		0		0
Derivative Liabilities, Fair Value, Net of Cash Collateral	12,000		843,000		713,000
Not Designated as Hedging Instrument [Member] | Foreign exchange contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	123,278,000	[1]	136,543,000	[1]	176,617,000	[1]
Derivative Liabilities, Notional	122,939,000	[1]	135,237,000	[1]	176,170,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	123,278,000		136,543,000		176,617,000
Derivative Assets, Netting Adjustments	0		0		0
Derivative Assets, Net Fair Value Before Cash Collateral	123,278,000		136,543,000		176,617,000
Derivative Asset, Cash Collateral	0		(2,147,000)		0
Derviative contracts, net of cash margin, assets	123,278,000		134,396,000		176,617,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	122,939,000		135,237,000		176,170,000
Derivative Liabilities, Netting Adjustments	0		0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral	122,939,000		135,237,000		176,170,000
Derivative Liabilities, Fair Value, Cash Collateral	0		(294,000)		0
Derivative Liabilities, Fair Value, Net of Cash Collateral	122,939,000		134,943,000		176,170,000
Not Designated as Hedging Instrument [Member] | Equity option contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	208,977,000	[1]	210,816,000	[1]	212,147,000	[1]
Derivative Liabilities, Notional	208,977,000	[1]	210,816,000	[1]	212,147,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	17,939,000		17,957,000		14,054,000
Derivative Assets, Netting Adjustments	0		0		0
Derivative Assets, Net Fair Value Before Cash Collateral	17,939,000		17,957,000		14,054,000
Derivative Asset, Cash Collateral	(3,380,000)		(3,472,000)		0
Derviative contracts, net of cash margin, assets	14,559,000		14,485,000		14,054,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	17,939,000		17,957,000		14,054,000
Derivative Liabilities, Netting Adjustments	0		0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral	17,939,000		17,957,000		14,054,000
Derivative Liabilities, Fair Value, Cash Collateral	0		0		0
Derivative Liabilities, Fair Value, Net of Cash Collateral	17,939,000		17,957,000		14,054,000
Not Designated as Hedging Instrument [Member] | Customer risk management programs [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	13,778,569,000	[1]	13,812,049,000	[1]	15,780,857,000	[1]
Derivative Liabilities, Notional	14,236,964,000	[1]	13,927,098,000	[1]	16,152,283,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	271,093,000		351,683,000		382,524,000
Derivative Assets, Netting Adjustments	(49,206,000)		(77,746,000)		(60,611,000)
Derivative Assets, Net Fair Value Before Cash Collateral	221,887,000		273,937,000		321,913,000
Derivative Asset, Cash Collateral	(3,380,000)		(8,925,000)		(1,622,000)
Derviative contracts, net of cash margin, assets	218,507,000		265,012,000		320,291,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	265,922,000		345,505,000		379,280,000
Derivative Liabilities, Netting Adjustments	(49,206,000)		(77,746,000)		(60,611,000)
Derivative Liabilities, Net Fair Value Before Cash Collateral	216,716,000		267,759,000		318,669,000
Derivative Liabilities, Fair Value, Cash Collateral	(33,877,000)		(24,202,000)		(67,217,000)
Derivative Liabilities, Fair Value, Net of Cash Collateral	182,839,000		243,557,000		251,452,000
Not Designated as Hedging Instrument [Member] | Interest rate risk management programs [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	0	[1]	0	[1]	22,000,000	[1]
Derivative Liabilities, Notional	47,000,000	[1]	47,000,000	[1]	25,000,000	[1]
Derivatives, Notional	47,000,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	0		0		182,000
Derivative Assets, Netting Adjustments	0		0		0
Derivative Assets, Net Fair Value Before Cash Collateral	0		0		182,000
Derivative Asset, Cash Collateral	0		0		0
Derviative contracts, net of cash margin, assets	0		0		182,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	2,660,000		3,628,000		384,000
Derivative Liabilities, Netting Adjustments	0		0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral	2,660,000		3,628,000		384,000
Derivative Liabilities, Fair Value, Cash Collateral	0		0		0
Derivative Liabilities, Fair Value, Net of Cash Collateral	$ 2,660,000		$ 3,628,000		$ 384,000

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