Derivatives, Fair Value of Derivatives Contracts (Details) - USD ($) $ in Thousands	Mar. 31, 2018		Dec. 31, 2017		Mar. 31, 2017
Derivative Assets, Fair Value, Net [Abstract]
Derviative contracts, net of cash collateral, Assets, Fair value	$ 286,687		$ 220,502		$ 304,727
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Fair Value, Net of Cash Collateral	233,202		171,963		276,422
Not Designated as Hedging Instrument [Member]
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	19,210,882	[1]	20,038,522	[2]	20,047,451	[3]
Derivative Liabilities, Notional	19,714,319	[1]	20,186,467	[2]	18,107,686	[3]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	448,683		301,052		371,995
Derivative Assets, Netting Adjustments	(152,065)		(74,022)		(62,384)
Derivative Assets, Net Fair Value Before Cash Collateral	296,618		227,030		309,611
Derivative Assets, Cash Collateral	(9,931)		(6,528)		(4,884)
Derviative contracts, net of cash collateral, Assets, Fair value	286,687		220,502		304,727
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	451,679		302,405		378,140
Derivative Liabilities, Netting Adjustments	(152,065)		(74,022)		(62,384)
Derivative Liabilities, Net Fair Value Before Cash Collateral	299,614		228,383		315,756
Derivative Liabilities, Cash Collateral	(66,412)		(56,420)		(39,334)
Derivative Liabilities, Fair Value, Net of Cash Collateral	233,202		171,963		276,422
Not Designated as Hedging Instrument [Member] | To-be-announced residential mortgage-backed securities
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	12,375,681	[1]	12,347,542	[2]	14,549,828	[3]
Derivative Liabilities, Notional	12,229,881	[1]	11,537,742	[2]	14,322,223	[3]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	49,371		23,606		80,272
Derivative Assets, Netting Adjustments	(36,833)		(18,096)		(33,143)
Derivative Assets, Net Fair Value Before Cash Collateral	12,538		5,510		47,129
Derivative Assets, Cash Collateral	0		0		0
Derviative contracts, net of cash collateral, Assets, Fair value	12,538		5,510		47,129
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	46,180		20,367		76,971
Derivative Liabilities, Netting Adjustments	(36,833)		(18,096)		(33,143)
Derivative Liabilities, Net Fair Value Before Cash Collateral	9,347		2,271		43,828
Derivative Liabilities, Cash Collateral	(6,539)		(704)		(34,310)
Derivative Liabilities, Fair Value, Net of Cash Collateral	2,808		1,567		9,518
Not Designated as Hedging Instrument [Member] | Interest rate swaps
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	1,500,421	[1]	1,478,944	[2]	1,491,414	[3]
Derivative Liabilities, Notional	1,500,421	[1]	1,478,944	[2]	1,491,414	[3]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	38,114		28,278		32,286
Derivative Assets, Netting Adjustments	(2,220)		0		0
Derivative Assets, Net Fair Value Before Cash Collateral	35,894		28,278		32,286
Derivative Assets, Cash Collateral	(9,107)		(4,964)		(3,349)
Derviative contracts, net of cash collateral, Assets, Fair value	26,787		23,314		28,937
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	38,116		28,298		33,036
Derivative Liabilities, Netting Adjustments	(2,220)		0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral	35,896		28,298		33,036
Derivative Liabilities, Cash Collateral	(5,674)		(12,896)		0
Derivative Liabilities, Fair Value, Net of Cash Collateral	30,222		15,402		33,036
Not Designated as Hedging Instrument [Member] | Energy contracts
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	1,219,505	[1]	1,190,067	[2]	886,699	[3]
Derivative Liabilities, Notional	1,193,851	[1]	1,166,924	[2]	844,406	[3]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	134,873		103,044		42,598
Derivative Assets, Netting Adjustments	(59,308)		(47,873)		(28,455)
Derivative Assets, Net Fair Value Before Cash Collateral	75,565		55,171		14,143
Derivative Assets, Cash Collateral	0		(196)		(543)
Derviative contracts, net of cash collateral, Assets, Fair value	75,565		54,975		13,600
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	131,459		101,603		40,604
Derivative Liabilities, Netting Adjustments	(59,308)		(47,873)		(28,455)
Derivative Liabilities, Net Fair Value Before Cash Collateral	72,151		53,730		12,149
Derivative Liabilities, Cash Collateral	(54,199)		(42,767)		(258)
Derivative Liabilities, Fair Value, Net of Cash Collateral	17,952		10,963		11,891
Not Designated as Hedging Instrument [Member] | Agricultural contracts
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	60,706	[1]	53,238	[2]	51,679	[3]
Derivative Liabilities, Notional	60,608	[1]	48,552	[2]	51,509	[3]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	2,023		1,576		2,031
Derivative Assets, Netting Adjustments	(1,256)		(960)		(786)
Derivative Assets, Net Fair Value Before Cash Collateral	767		616		1,245
Derivative Assets, Cash Collateral	0		0		0
Derviative contracts, net of cash collateral, Assets, Fair value	767		616		1,245
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	1,993		1,551		2,015
Derivative Liabilities, Netting Adjustments	(1,256)		(960)		(786)
Derivative Liabilities, Net Fair Value Before Cash Collateral	737		591		1,229
Derivative Liabilities, Cash Collateral	0		0		(1,040)
Derivative Liabilities, Fair Value, Net of Cash Collateral	737		591		189
Not Designated as Hedging Instrument [Member] | Foreign exchange contracts
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	166,060	[1]	132,397	[2]	211,837	[3]
Derivative Liabilities, Notional	160,945	[1]	126,251	[2]	208,236	[3]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	162,966		129,551		204,774
Derivative Assets, Netting Adjustments	0		0		0
Derivative Assets, Net Fair Value Before Cash Collateral	162,966		129,551		204,774
Derivative Assets, Cash Collateral	(164)		(448)		(72)
Derviative contracts, net of cash collateral, Assets, Fair value	162,802		129,103		204,702
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	157,621		123,321		201,043
Derivative Liabilities, Netting Adjustments	0		0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral	157,621		123,321		201,043
Derivative Liabilities, Cash Collateral	0		(53)		(3,726)
Derivative Liabilities, Fair Value, Net of Cash Collateral	157,621		123,268		197,317
Not Designated as Hedging Instrument [Member] | Equity option contracts
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	99,239	[1]	99,633	[2]	99,031	[3]
Derivative Liabilities, Notional	99,239	[1]	99,633	[2]	99,031	[3]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	3,949		5,503		4,505
Derivative Assets, Netting Adjustments	0		0		0
Derivative Assets, Net Fair Value Before Cash Collateral	3,949		5,503		4,505
Derivative Assets, Cash Collateral	(660)		(920)		(920)
Derviative contracts, net of cash collateral, Assets, Fair value	3,289		4,583		3,585
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	3,949		5,503		4,505
Derivative Liabilities, Netting Adjustments	0		0		0
Derivative Liabilities, Net Fair Value Before Cash Collateral	3,949		5,503		4,505
Derivative Liabilities, Cash Collateral	0		0		0
Derivative Liabilities, Fair Value, Net of Cash Collateral	3,949		5,503		4,505
Not Designated as Hedging Instrument [Member] | Total customer risk management programs
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	15,421,612	[1]	15,301,821	[2]	17,290,488	[3]
Derivative Liabilities, Notional	15,244,945	[1]	14,458,046	[2]	17,016,819	[3]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	391,296		291,558		366,466
Derivative Assets, Netting Adjustments	(99,617)		(66,929)		(62,384)
Derivative Assets, Net Fair Value Before Cash Collateral	291,679		224,629		304,082
Derivative Assets, Cash Collateral	(9,931)		(6,528)		(4,884)
Derviative contracts, net of cash collateral, Assets, Fair value	281,748		218,101		299,198
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	379,318		280,643		358,174
Derivative Liabilities, Netting Adjustments	(99,617)		(66,929)		(62,384)
Derivative Liabilities, Net Fair Value Before Cash Collateral	279,701		213,714		295,790
Derivative Liabilities, Cash Collateral	(66,412)		(56,420)		(39,334)
Derivative Liabilities, Fair Value, Net of Cash Collateral	213,289		157,294		256,456
Not Designated as Hedging Instrument [Member] | Internal risk management programs
Notional Amount of Derivatives [Abstract]
Derivative Assets, Notional	3,789,270	[1]	4,736,701	[2]	2,756,963	[3]
Derivative Liabilities, Notional	4,469,374	[1]	5,728,421	[2]	1,090,867	[3]
Derivative Assets, Fair Value, Net [Abstract]
Derivative Assets, Gross Fair Value	57,387		9,494		5,529
Derivative Assets, Netting Adjustments	(52,448)		(7,093)		0
Derivative Assets, Net Fair Value Before Cash Collateral	4,939		2,401		5,529
Derivative Assets, Cash Collateral	0		0		0
Derviative contracts, net of cash collateral, Assets, Fair value	4,939		2,401		5,529
Derivative Liabilities, Fair Value, Net [Abstract]
Derivative Liabilities, Gross Fair Value	72,361		21,762		19,966
Derivative Liabilities, Netting Adjustments	(52,448)		(7,093)		0
Derivative Liabilities, Net Fair Value Before Cash Collateral	19,913		14,669		19,966
Derivative Liabilities, Cash Collateral	0		0		0
Derivative Liabilities, Fair Value, Net of Cash Collateral	$ 19,913		$ 14,669		$ 19,966

[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.