Derivatives, Fair Value of Derivatives Contracts (Details) (USD $)	Mar. 31, 2015		Dec. 31, 2014		Mar. 31, 2014
Derivative Assets, Fair Value, Net [Abstract]
Derviative contracts, net of cash margin, Assets, Fair value	$ 462,386,000		$ 361,874,000		$ 218,507,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Fair Value, Net of Cash Collateral	419,351,000		354,554,000		185,499,000
Increase in cash margin obligation for derivative holdings with a decrease in credit rating from A1 to below investment grade	22,000,000
Not Designated as Hedging Instrument [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	20,594,556,000	[1]	15,592,566,000	[1]	13,778,569,000	[1]
Derivative Liabilities, Notional	20,355,523,000	[1]	15,797,483,000	[1]	14,283,964,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	610,682,000		522,423,000		271,093,000
Derivative Assets, Netting Adjustments	(86,078,000)		(89,239,000)		(49,206,000)
Derivative Assets, Net Fair Value Before Cash Collateral	524,604,000		433,184,000		221,887,000
Derivative Assets, Cash Collateral	(62,218,000)		(71,310,000)		(3,380,000)
Derviative contracts, net of cash margin, Assets, Fair value	462,386,000		361,874,000		218,507,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	603,360,000		522,099,000		268,582,000
Derivative Liabilities, Netting Adjustments	(86,078,000)		(89,239,000)		(49,206,000)
Derivative Liabilities, Net Fair Value Before Cash Collateral	517,282,000		432,860,000		219,376,000
Derivative Liabilities, Cash Collateral	(97,931,000)		(78,306,000)		(33,877,000)
Derivative Liabilities, Fair Value, Net of Cash Collateral	419,351,000		354,554,000		185,499,000
Not Designated as Hedging Instrument [Member] | To-be-announced residential mortgage-backed securities [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	18,144,202,000	[1]	13,313,615,000	[1]	10,859,613,000	[1]
Derivative Liabilities, Notional	17,920,104,000	[1]	13,471,880,000	[1]	11,398,442,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	115,693,000		94,719,000		30,897,000
Derivative Assets, Netting Adjustments	(38,135,000)		(39,359,000)		(20,219,000)
Derivative Assets, Net Fair Value Before Cash Collateral	77,558,000		55,360,000		10,678,000
Derivative Assets, Cash Collateral	0		0		0
Derviative contracts, net of cash margin, Assets, Fair value	77,558,000		55,360,000		10,678,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	111,977,000		91,949,000		27,966,000
Derivative Liabilities, Netting Adjustments	(38,135,000)		(39,359,000)		(20,219,000)
Derivative Liabilities, Net Fair Value Before Cash Collateral	73,842,000		52,590,000		7,747,000
Derivative Liabilities, Cash Collateral	(61,094,000)		(52,290,000)		0
Derivative Liabilities, Fair Value, Net of Cash Collateral	12,748,000		300,000		7,747,000
Not Designated as Hedging Instrument [Member] | Interest rate swaps [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	1,174,975,000	[1]	1,165,568,000	[1]	1,266,880,000	[1]
Derivative Liabilities, Notional	1,174,975,000	[1]	1,165,568,000	[1]	1,266,880,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	39,880,000		35,405,000		41,331,000
Derivative Assets, Netting Adjustments	0		0		0
Derivative Assets, Net Fair Value Before Cash Collateral	39,880,000		35,405,000		41,331,000
Derivative Assets, Cash Collateral	0		0		0
Derviative contracts, net of cash margin, Assets, Fair value	39,880,000		35,405,000		41,331,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	40,134,000		35,599,000		41,596,000
Derivative Liabilities, Netting Adjustments	0		0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral	40,134,000		35,599,000		41,596,000
Derivative Liabilities, Cash Collateral	(23,121,000)		(18,717,000)		(17,388,000)
Derivative Liabilities, Fair Value, Net of Cash Collateral	17,013,000		16,882,000		24,208,000
Not Designated as Hedging Instrument [Member] | Energy contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	651,548,000	[1]	579,801,000	[1]	1,207,861,000	[1]
Derivative Liabilities, Notional	634,459,000	[1]	579,801,000	[1]	1,134,208,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	133,391,000		141,166,000		53,440,000
Derivative Assets, Netting Adjustments	(47,576,000)		(48,624,000)		(27,112,000)
Derivative Assets, Net Fair Value Before Cash Collateral	85,815,000		92,542,000		26,328,000
Derivative Assets, Cash Collateral	(62,118,000)		(71,310,000)		0
Derviative contracts, net of cash margin, Assets, Fair value	23,697,000		21,232,000		26,328,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	130,396,000		142,839,000		51,308,000
Derivative Liabilities, Netting Adjustments	(47,576,000)		(48,624,000)		(27,112,000)
Derivative Liabilities, Net Fair Value Before Cash Collateral	82,820,000		94,215,000		24,196,000
Derivative Liabilities, Cash Collateral	0		0		(14,202,000)
Derivative Liabilities, Fair Value, Net of Cash Collateral	82,820,000		94,215,000		9,994,000
Not Designated as Hedging Instrument [Member] | Agricultural contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	37,545,000	[1]	47,657,000	[1]	111,960,000	[1]
Derivative Liabilities, Notional	37,536,000	[1]	47,418,000	[1]	105,518,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	837,000		1,904,000		4,208,000
Derivative Assets, Netting Adjustments	(367,000)		(1,256,000)		(1,875,000)
Derivative Assets, Net Fair Value Before Cash Collateral	470,000		648,000		2,333,000
Derivative Assets, Cash Collateral	0		0		0
Derviative contracts, net of cash margin, Assets, Fair value	470,000		648,000		2,333,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	830,000		1,908,000		4,174,000
Derivative Liabilities, Netting Adjustments	(367,000)		(1,256,000)		(1,875,000)
Derivative Liabilities, Net Fair Value Before Cash Collateral	463,000		652,000		2,299,000
Derivative Liabilities, Cash Collateral	0		(596,000)		(2,287,000)
Derivative Liabilities, Fair Value, Net of Cash Collateral	463,000		56,000		12,000
Not Designated as Hedging Instrument [Member] | Foreign exchange contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	379,243,000	[1]	290,965,000	[1]	123,278,000	[1]
Derivative Liabilities, Notional	378,406,000	[1]	290,856,000	[1]	122,939,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	311,739,000		238,395,000		123,278,000
Derivative Assets, Netting Adjustments	0		0		0
Derivative Assets, Net Fair Value Before Cash Collateral	311,739,000		238,395,000		123,278,000
Derivative Assets, Cash Collateral	0		0		0
Derviative contracts, net of cash margin, Assets, Fair value	311,739,000		238,395,000		123,278,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	310,940,000		238,118,000		122,939,000
Derivative Liabilities, Netting Adjustments	0		0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral	310,940,000		238,118,000		122,939,000
Derivative Liabilities, Cash Collateral	(13,716,000)		(6,703,000)		0
Derivative Liabilities, Fair Value, Net of Cash Collateral	297,224,000		231,415,000		122,939,000
Not Designated as Hedging Instrument [Member] | Equity option contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	185,043,000	[1]	194,960,000	[1]	208,977,000	[1]
Derivative Liabilities, Notional	185,043,000	[1]	194,960,000	[1]	208,977,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	8,939,000		10,834,000		17,939,000
Derivative Assets, Netting Adjustments	0		0		0
Derivative Assets, Net Fair Value Before Cash Collateral	8,939,000		10,834,000		17,939,000
Derivative Assets, Cash Collateral	(100,000)		0		(3,380,000)
Derviative contracts, net of cash margin, Assets, Fair value	8,839,000		10,834,000		14,559,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	8,939,000		10,834,000		17,939,000
Derivative Liabilities, Netting Adjustments	0		0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral	8,939,000		10,834,000		17,939,000
Derivative Liabilities, Cash Collateral	0		0		0
Derivative Liabilities, Fair Value, Net of Cash Collateral	8,939,000		10,834,000		17,939,000
Not Designated as Hedging Instrument [Member] | Total customer risk management programs [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	20,572,556,000	[1]	15,592,566,000	[1]	13,778,569,000	[1]
Derivative Liabilities, Notional	20,330,523,000	[1]	15,750,483,000	[1]	14,236,964,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	610,479,000		522,423,000		271,093,000
Derivative Assets, Netting Adjustments	(86,078,000)		(89,239,000)		(49,206,000)
Derivative Assets, Net Fair Value Before Cash Collateral	524,401,000		433,184,000		221,887,000
Derivative Assets, Cash Collateral	(62,218,000)		(71,310,000)		(3,380,000)
Derviative contracts, net of cash margin, Assets, Fair value	462,183,000		361,874,000		218,507,000
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	603,216,000		521,247,000		265,922,000
Derivative Liabilities, Netting Adjustments	(86,078,000)		(89,239,000)		(49,206,000)
Derivative Liabilities, Net Fair Value Before Cash Collateral	517,138,000		432,008,000		216,716,000
Derivative Liabilities, Cash Collateral	(97,931,000)		(78,306,000)		(33,877,000)
Derivative Liabilities, Fair Value, Net of Cash Collateral	419,207,000		353,702,000		182,839,000
Not Designated as Hedging Instrument [Member] | Interest rate risk management programs [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	22,000,000	[1]	0	[1]	0	[1]
Derivative Liabilities, Notional	25,000,000	[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	203,000		0		0
Derivative Assets, Netting Adjustments	0		0		0
Derivative Assets, Net Fair Value Before Cash Collateral	203,000		0		0
Derivative Assets, Cash Collateral	0		0		0
Derviative contracts, net of cash margin, Assets, Fair value	203,000		0		0
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	144,000		852,000		2,660,000
Derivative Liabilities, Netting Adjustments	0		0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral	144,000		852,000		2,660,000
Derivative Liabilities, Cash Collateral	0		0		0
Derivative Liabilities, Fair Value, Net of Cash Collateral	$ 144,000		$ 852,000		$ 2,660,000

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