Derivatives (Narrative) (Details) MMBTU in Thousands, Boe in Thousands, $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019 USD ($) MMBTU Boe company	Dec. 31, 2018 USD ($) MMBTU Boe company
Derivative [Line Items]
Derivative liabilities - Fair value	$ 109,587	$ 118,305
Derivative assets - Fair value	135,782	107,816
Credit-risk-related contingent features
Derivative [Line Items]
Derivative asset	587	2,800
Derivative liability	27,900	14,200
Collateral Already Posted Aggregated, Fair Value	27,100	9,400
Aggregate fair value of derivative instruments in net liability position	27,300	11,400
Derivative instruments designated as hedging instruments
Derivative [Line Items]
Notional amount	123,241	126,056
Derivative liabilities - Fair value	4,678	6,477
Derivative assets - Fair value	0	0
Derivative instruments designated as hedging instruments \| Interest rate contracts
Derivative [Line Items]
Notional amount	31,026	35,811
Derivative liabilities - Fair value	4,660	5,866
Derivative assets - Fair value	0	0
Derivative instruments designated as hedging instruments \| Foreign exchange contracts
Derivative [Line Items]
Notional amount	92,215	90,245
Derivative liabilities - Fair value	18	611
Derivative assets - Fair value	0	0
Derivatives not designated as hedging instruments
Derivative [Line Items]
Notional amount	15,873,400	15,222,341
Derivative liabilities - Fair value	104,909	111,828
Derivative assets - Fair value	135,782	107,816
Derivatives not designated as hedging instruments \| Interest rate contracts
Derivative [Line Items]
Notional amount	12,266,761	11,695,499
Derivative liabilities - Fair value	71,912	69,267
Derivative assets - Fair value	96,256	69,818
Derivatives not designated as hedging instruments \| Interest rate contracts \| LCH
Derivative [Line Items]
Notional amount	1,820,000	1,660,000
Derivative asset	7,400	16,400
Derivative liability	32,800	16,000
Derivatives not designated as hedging instruments \| Interest rate contracts \| Financial Counterparty
Derivative [Line Items]
Notional amount	6,140,000	5,850,000
Derivatives not designated as hedging instruments \| Foreign exchange contracts
Derivative [Line Items]
Notional amount	3,513,714	3,407,522
Derivative liabilities - Fair value	24,900	19,329
Derivative assets - Fair value	30,085	21,624
Derivatives not designated as hedging instruments \| Credit contracts
Derivative [Line Items]
Notional amount	92,925	119,320
Derivative liabilities - Fair value	$ 81	$ 164
Weighted average remaining maturity of outstanding RPAs	4 years 10 months 24 days	6 years 7 months 6 days
Derivative assets - Fair value	$ 1	$ 1
Derivatives not designated as hedging instruments \| Credit contracts \| RPA protection sold
Derivative [Line Items]
Maximum exposure of RPAs with protection sold	$ 84	$ 125
Derivatives not designated as hedging instruments \| Equity, Public Companies
Derivative [Line Items]
Number of companies that issued the equity (issuers portion only) \| company	4	4
Derivatives not designated as hedging instruments \| Equity, Private Companies
Derivative [Line Items]
Number of companies that issued the equity (issuers portion only) \| company	17	18
Derivatives not designated as hedging instruments \| Equity contracts
Derivative [Line Items]
Notional amount	$ 0	$ 0
Derivative liabilities - Fair value	0	0
Derivative assets - Fair value	2,201	1,951
Derivatives not designated as hedging instruments \| Commodity contracts
Derivative [Line Items]
Notional amount	0	0
Derivative liabilities - Fair value	8,016	23,068
Derivative assets - Fair value	7,239	14,422
Derivatives not designated as hedging instruments \| Commodity contracts \| Chicago Mercantile Exchange (CME)
Derivative [Line Items]
Derivative asset	2,100	10,400
Derivative liability	720	582
Derivative assets (liabilities), at fair value, net	$ (12)	$ 622
Derivatives not designated as hedging instruments \| Commodity contracts \| Chicago Mercantile Exchange (CME) \| Oil
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure \| Boe	1,028	778
Derivatives not designated as hedging instruments \| Commodity contracts \| Chicago Mercantile Exchange (CME) \| Natural gas
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure \| MMBTU	6,903	6,290
Maximum \| Derivatives not designated as hedging instruments \| Foreign exchange contracts
Derivative [Line Items]
Original maturity (in years)	1 year	1 year

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Number of Companies that Issued the Warrants (Issuers Portion Only)

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Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

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-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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Maximum potential amount of future undiscounted payments that could be required under the credit derivative, before reduction for potential recoveries under recourse or collateralization provisions.

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Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

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Average remaining period until maturity of the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Nominal energy measure used to calculate payments on a derivative instrument.

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