Derivatives, Fair Value of Derivatives Contracts (Details) - USD ($) $ in Thousands		Mar. 31, 2016		Dec. 31, 2015		Mar. 31, 2015
Derivative Assets, Fair Value, Net [Abstract]
Derviative contracts, net of cash collateral, Assets, Fair value		$ 790,146		$ 586,270		$ 462,386
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Fair Value, Net of Cash Collateral		705,578		581,701		419,351
Increase in cash margin obligation for derivative holdings with a decrease in credit rating from A1 to below investment grade		18,000
Not Designated as Hedging Instrument [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	[1]	19,792,138		17,153,221		20,594,556
Derivative Liabilities, Notional		19,101,068	[2]	16,785,014	[1]	20,355,523	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value		873,951		663,343		610,682
Derivative Assets, Netting Adjustments		(67,119)		(52,433)		(86,078)
Derivative Assets, Net Fair Value Before Cash Collateral		806,832		610,910		524,604
Derivative Assets, Cash Collateral		(16,686)		(24,640)		(62,218)
Derviative contracts, net of cash collateral, Assets, Fair value		790,146		586,270		462,386
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value		863,433		659,171		603,360
Derivative Liabilities, Netting Adjustments		(67,119)		(52,433)		(86,078)
Derivative Liabilities, Net Fair Value Before Cash Collateral		796,314		606,738		517,282
Derivative Liabilities, Cash Collateral		(90,736)		(25,037)		(97,931)
Derivative Liabilities, Fair Value, Net of Cash Collateral		705,578		581,701		419,351
Not Designated as Hedging Instrument [Member] | To-be-announced residential mortgage-backed securities [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	[1]	16,235,517		14,583,052		18,144,202
Derivative Liabilities, Notional		16,323,807	[2]	14,168,927	[3]	17,920,104	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value		111,188		43,270		115,693
Derivative Assets, Netting Adjustments		(44,570)		(28,305)		(38,135)
Derivative Assets, Net Fair Value Before Cash Collateral		66,618		14,965		77,558
Derivative Assets, Cash Collateral		0		0		0
Derviative contracts, net of cash collateral, Assets, Fair value		66,618		14,965		77,558
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value		107,541		40,141		111,977
Derivative Liabilities, Netting Adjustments		(44,570)		(28,305)		(38,135)
Derivative Liabilities, Net Fair Value Before Cash Collateral		62,971		11,836		73,842
Derivative Liabilities, Cash Collateral		(61,380)		(1,308)		(61,094)
Derivative Liabilities, Fair Value, Net of Cash Collateral		1,591		10,528		12,748
Not Designated as Hedging Instrument [Member] | Interest rate swaps [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	[3]	1,335,259		1,332,044		1,174,975
Derivative Liabilities, Notional		1,335,259	[2]	1,332,044	[3]	1,174,975	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value		48,270		31,744		39,880
Derivative Assets, Netting Adjustments		0		0		0
Derivative Assets, Net Fair Value Before Cash Collateral		48,270		31,744		39,880
Derivative Assets, Cash Collateral		0		(1,424)		0
Derviative contracts, net of cash collateral, Assets, Fair value		48,270		30,320		39,880
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value		48,619		31,928		40,134
Derivative Liabilities, Netting Adjustments		0		0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral		48,619		31,928		40,134
Derivative Liabilities, Cash Collateral		(28,572)		(20,530)		(23,121)
Derivative Liabilities, Fair Value, Net of Cash Collateral		20,047		11,398		17,013
Not Designated as Hedging Instrument [Member] | Energy contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	[1]	533,355		470,613		651,548
Derivative Liabilities, Notional		526,103	[2]	463,703	[3]	634,459	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value		62,365		83,045		133,391
Derivative Assets, Netting Adjustments		(21,374)		(22,970)		(47,576)
Derivative Assets, Net Fair Value Before Cash Collateral		40,991		60,075		85,815
Derivative Assets, Cash Collateral		(11,340)		(18,606)		(62,118)
Derviative contracts, net of cash collateral, Assets, Fair value		29,651		41,469		23,697
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value		62,528		81,869		130,396
Derivative Liabilities, Netting Adjustments		(21,374)		(22,970)		(47,576)
Derivative Liabilities, Net Fair Value Before Cash Collateral		41,154		58,899		82,820
Derivative Liabilities, Cash Collateral		0		0		0
Derivative Liabilities, Fair Value, Net of Cash Collateral		41,154		58,899		82,820
Not Designated as Hedging Instrument [Member] | Agricultural contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	[1]	104,927		61,662		37,545
Derivative Liabilities, Notional		104,922	[2]	61,657	[3]	37,536	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value		1,859		2,591		837
Derivative Assets, Netting Adjustments		(1,175)		(1,158)		(367)
Derivative Assets, Net Fair Value Before Cash Collateral		684		1,433		470
Derivative Assets, Cash Collateral		0		0		0
Derviative contracts, net of cash collateral, Assets, Fair value		684		1,433		470
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value		1,847		2,579		830
Derivative Liabilities, Netting Adjustments		(1,175)		(1,158)		(367)
Derivative Liabilities, Net Fair Value Before Cash Collateral		672		1,421		463
Derivative Liabilities, Cash Collateral		(420)		(1,248)		0
Derivative Liabilities, Fair Value, Net of Cash Collateral		252		173		463
Not Designated as Hedging Instrument [Member] | Foreign exchange contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	[1]	682,457		546,572		379,243
Derivative Liabilities, Notional		682,354	[2]	546,405	[3]	378,406	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value		639,322		498,830		311,739
Derivative Assets, Netting Adjustments		0		0		0
Derivative Assets, Net Fair Value Before Cash Collateral		639,322		498,830		311,739
Derivative Assets, Cash Collateral		(4,970)		(4,140)		0
Derviative contracts, net of cash collateral, Assets, Fair value		634,352		494,690		311,739
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value		638,892		498,574		310,940
Derivative Liabilities, Netting Adjustments		0		0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral		638,892		498,574		310,940
Derivative Liabilities, Cash Collateral		(364)		(1,951)		(13,716)
Derivative Liabilities, Fair Value, Net of Cash Collateral		638,528		496,623		297,224
Not Designated as Hedging Instrument [Member] | Equity option contracts [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	[1]	128,623		137,278		185,043
Derivative Liabilities, Notional		128,623	[2]	137,278	[3]	185,043	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value		4,006		3,780		8,939
Derivative Assets, Netting Adjustments		0		0		0
Derivative Assets, Net Fair Value Before Cash Collateral		4,006		3,780		8,939
Derivative Assets, Cash Collateral		(376)		(470)		(100)
Derviative contracts, net of cash collateral, Assets, Fair value		3,630		3,310		8,839
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value		4,006		3,780		8,939
Derivative Liabilities, Netting Adjustments		0		0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral		4,006		3,780		8,939
Derivative Liabilities, Cash Collateral		0		0		0
Derivative Liabilities, Fair Value, Net of Cash Collateral		4,006		3,780		8,939
Not Designated as Hedging Instrument [Member] | Total customer risk management programs [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	[1]	19,020,138		17,131,221		20,572,556
Derivative Liabilities, Notional		19,101,068	[2]	16,710,014	[3]	20,330,523	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value		867,010		663,260		610,479
Derivative Assets, Netting Adjustments		(67,119)		(52,433)		(86,078)
Derivative Assets, Net Fair Value Before Cash Collateral		799,891		610,827		524,401
Derivative Assets, Cash Collateral		(16,686)		(24,640)		(62,218)
Derviative contracts, net of cash collateral, Assets, Fair value		783,205		586,187		462,183
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value		863,433		658,871		603,216
Derivative Liabilities, Netting Adjustments		(67,119)		(52,433)		(86,078)
Derivative Liabilities, Net Fair Value Before Cash Collateral		796,314		606,438		517,138
Derivative Liabilities, Cash Collateral		(90,736)		(25,037)		(97,931)
Derivative Liabilities, Fair Value, Net of Cash Collateral		705,578		581,401		419,207
Not Designated as Hedging Instrument [Member] | Interest rate risk management programs [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	[1]	772,000		22,000		22,000
Derivative Liabilities, Notional		0	[2]	75,000	[3]	25,000	[1]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value		6,941		83		203
Derivative Assets, Netting Adjustments		0		0		0
Derivative Assets, Net Fair Value Before Cash Collateral		6,941		83		203
Derivative Assets, Cash Collateral		0		0		0
Derviative contracts, net of cash collateral, Assets, Fair value		6,941		83		203
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value		0		300		144
Derivative Liabilities, Netting Adjustments		0		0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral		0		300		144
Derivative Liabilities, Cash Collateral		0		0		0
Derivative Liabilities, Fair Value, Net of Cash Collateral		$ 0		$ 300		$ 144

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

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

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