Derivatives, Fair Value of Derivatives Contracts (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013
Derivative Assets, Fair Value, Net [Abstract]
Derivative contracts, net of cash margin, Assets, Fair Value	$ 361,874,000		$ 265,012,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative contracts, net of cash margin, Liabilities, Fair Value	354,554,000		247,185,000
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	15,592,566,000	[1]	13,812,049,000	[1]
Derivative Liabilities, Notional	15,797,483,000	[1]	13,974,098,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	522,423,000		351,683,000
Derivative Assets, Netting Adjustments	(89,239,000)		(77,746,000)
Derivative Assets, Net Fair Value Before Cash Collateral	433,184,000		273,937,000
Derivative Assets, Fair Value, Cash Collateral	(71,310,000)		(8,925,000)
Derivative contracts, net of cash margin, Assets, Fair Value	361,874,000		265,012,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Fair Value, Gross Liability	522,099,000		349,133,000
Derivative Liabilities, Netting Adjustments	(89,239,000)		(77,746,000)
Derivative Liabilities, Net Fair Value Before Cash Collateral	432,860,000		271,387,000
Derivative Liabilities, Fair Value, Cash Collateral	(78,306,000)		(24,202,000)
Derivative contracts, net of cash margin, Liabilities, Fair Value	354,554,000		247,185,000
Not Designated as Hedging Instrument [Member] | To-be-announced residential mortgage-backed securities [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	13,313,615,000	[1]	10,817,159,000	[1]
Derivative Liabilities, Notional	13,471,880,000	[1]	10,982,049,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	94,719,000		102,921,000
Derivative Assets, Netting Adjustments	(39,359,000)		(46,623,000)
Derivative Assets, Net Fair Value Before Cash Collateral	55,360,000		56,298,000
Derivative Assets, Fair Value, Cash Collateral	0		0
Derivative contracts, net of cash margin, Assets, Fair Value	55,360,000		56,298,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Fair Value, Gross Liability	91,949,000		99,830,000
Derivative Liabilities, Netting Adjustments	(39,359,000)		(46,623,000)
Derivative Liabilities, Net Fair Value Before Cash Collateral	52,590,000		53,207,000
Derivative Liabilities, Fair Value, Cash Collateral	(52,290,000)		0
Derivative contracts, net of cash margin, Liabilities, Fair Value	300,000		53,207,000
Not Designated as Hedging Instrument [Member] | Interest rate swaps [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	1,165,568,000	[1]	1,283,379,000	[1]
Derivative Liabilities, Notional	1,165,568,000	[1]	1,283,379,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	35,405,000		44,124,000
Derivative Assets, Netting Adjustments	0		0
Derivative Assets, Net Fair Value Before Cash Collateral	35,405,000		44,124,000
Derivative Assets, Fair Value, Cash Collateral	0		(731,000)
Derivative contracts, net of cash margin, Assets, Fair Value	35,405,000		43,393,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Fair Value, Gross Liability	35,599,000		44,377,000
Derivative Liabilities, Netting Adjustments	0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral	35,599,000		44,377,000
Derivative Liabilities, Fair Value, Cash Collateral	(18,717,000)		(17,853,000)
Derivative contracts, net of cash margin, Liabilities, Fair Value	16,882,000		26,524,000
Not Designated as Hedging Instrument [Member] | Energy contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	579,801,000	[1]	1,263,266,000	[1]
Derivative Liabilities, Notional	579,801,000	[1]	1,216,426,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	141,166,000		48,078,000
Derivative Assets, Netting Adjustments	(48,624,000)		(29,957,000)
Derivative Assets, Net Fair Value Before Cash Collateral	92,542,000		18,121,000
Derivative Assets, Fair Value, Cash Collateral	(71,310,000)		(2,575,000)
Derivative contracts, net of cash margin, Assets, Fair Value	21,232,000		15,546,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Fair Value, Gross Liability	142,839,000		46,095,000
Derivative Liabilities, Netting Adjustments	(48,624,000)		(29,957,000)
Derivative Liabilities, Net Fair Value Before Cash Collateral	94,215,000		16,138,000
Derivative Liabilities, Fair Value, Cash Collateral	0		(6,055,000)
Derivative contracts, net of cash margin, Liabilities, Fair Value	94,215,000		10,083,000
Not Designated as Hedging Instrument [Member] | Agricultural contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	47,657,000	[1]	100,886,000	[1]
Derivative Liabilities, Notional	47,418,000	[1]	99,191,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	1,904,000		2,060,000
Derivative Assets, Netting Adjustments	(1,256,000)		(1,166,000)
Derivative Assets, Net Fair Value Before Cash Collateral	648,000		894,000
Derivative Assets, Fair Value, Cash Collateral	0		0
Derivative contracts, net of cash margin, Assets, Fair Value	648,000		894,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Fair Value, Gross Liability	1,908,000		2,009,000
Derivative Liabilities, Netting Adjustments	(1,256,000)		(1,166,000)
Derivative Liabilities, Net Fair Value Before Cash Collateral	652,000		843,000
Derivative Liabilities, Fair Value, Cash Collateral	(596,000)		0
Derivative contracts, net of cash margin, Liabilities, Fair Value	56,000		843,000
Not Designated as Hedging Instrument [Member] | Foreign exchange contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	290,965,000	[1]	136,543,000	[1]
Derivative Liabilities, Notional	290,856,000	[1]	135,237,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	238,395,000		136,543,000
Derivative Assets, Netting Adjustments	0		0
Derivative Assets, Net Fair Value Before Cash Collateral	238,395,000		136,543,000
Derivative Assets, Fair Value, Cash Collateral	0		(2,147,000)
Derivative contracts, net of cash margin, Assets, Fair Value	238,395,000		134,396,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Fair Value, Gross Liability	238,118,000		135,237,000
Derivative Liabilities, Netting Adjustments	0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral	238,118,000		135,237,000
Derivative Liabilities, Fair Value, Cash Collateral	(6,703,000)		(294,000)
Derivative contracts, net of cash margin, Liabilities, Fair Value	231,415,000		134,943,000
Not Designated as Hedging Instrument [Member] | Equity option contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	194,960,000	[1]	210,816,000	[1]
Derivative Liabilities, Notional	194,960,000	[1]	210,816,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	10,834,000		17,957,000
Derivative Assets, Netting Adjustments	0		0
Derivative Assets, Net Fair Value Before Cash Collateral	10,834,000		17,957,000
Derivative Assets, Fair Value, Cash Collateral	0		(3,472,000)
Derivative contracts, net of cash margin, Assets, Fair Value	10,834,000		14,485,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Fair Value, Gross Liability	10,834,000		17,957,000
Derivative Liabilities, Netting Adjustments	0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral	10,834,000		17,957,000
Derivative Liabilities, Fair Value, Cash Collateral	0		0
Derivative contracts, net of cash margin, Liabilities, Fair Value	10,834,000		17,957,000
Not Designated as Hedging Instrument [Member] | Customer risk management programs [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	15,592,566,000	[1]	13,812,049,000	[1]
Derivative Liabilities, Notional	15,750,483,000	[1]	13,927,098,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	522,423,000		351,683,000
Derivative Assets, Netting Adjustments	(89,239,000)		(77,746,000)
Derivative Assets, Net Fair Value Before Cash Collateral	433,184,000		273,937,000
Derivative Assets, Fair Value, Cash Collateral	(71,310,000)		(8,925,000)
Derivative contracts, net of cash margin, Assets, Fair Value	361,874,000		265,012,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Fair Value, Gross Liability	521,247,000		345,505,000
Derivative Liabilities, Netting Adjustments	(89,239,000)		(77,746,000)
Derivative Liabilities, Net Fair Value Before Cash Collateral	432,008,000		267,759,000
Derivative Liabilities, Fair Value, Cash Collateral	(78,306,000)		(24,202,000)
Derivative contracts, net of cash margin, Liabilities, Fair Value	353,702,000		243,557,000
Not Designated as Hedging Instrument [Member] | Interest rate risk management programs [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	0	[1]	0	[1]
Derivative Liabilities, Notional	47,000,000	[1]	47,000,000	[1]
Derivative, Notional Amount	47,000,000
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	0		0
Derivative Assets, Netting Adjustments	0		0
Derivative Assets, Net Fair Value Before Cash Collateral	0		0
Derivative Assets, Fair Value, Cash Collateral	0		0
Derivative contracts, net of cash margin, Assets, Fair Value	0		0
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Fair Value, Gross Liability	852,000		3,628,000
Derivative Liabilities, Netting Adjustments	0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral	852,000		3,628,000
Derivative Liabilities, Fair Value, Cash Collateral	0		0
Derivative contracts, net of cash margin, Liabilities, Fair Value	$ 852,000		$ 3,628,000

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