Derivatives (Narrative) (Details) MMBTU in Thousands, Boe in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2019 USD ($) Boe MMBTU company	Dec. 31, 2018 USD ($) Boe MMBTU company
Derivative [Line Items]
Derivative assets - Fair value	$ 358,639	$ 107,816
Derivative liabilities - Fair value	263,734	118,305
Credit-risk-related contingent features
Derivative [Line Items]
Aggregate fair value of derivative instruments in net liability position	71,300	11,400
Derivative asset	538	2,800
Derivative liability	71,900	14,200
Collateral posted	71,100	9,400
Derivative instruments designated as hedging instruments
Derivative [Line Items]
Notional amount	115,857	126,056
Derivative assets - Fair value	0	0
Derivative liabilities - Fair value	3,235	6,477
Derivative instruments designated as hedging instruments \| Foreign exchange contracts
Derivative [Line Items]
Notional amount	84,831	90,245
Derivative assets - Fair value	0	0
Derivative liabilities - Fair value	431	611
Derivative instruments designated as hedging instruments \| Interest rate contracts
Derivative [Line Items]
Notional amount	31,026	35,811
Derivative assets - Fair value	0	0
Derivative liabilities - Fair value	2,804	5,866
Derivatives not designated as hedging instruments
Derivative [Line Items]
Notional amount	18,988,712	15,222,341
Derivative assets - Fair value	358,639	107,816
Derivative liabilities - Fair value	260,499	111,828
Derivatives not designated as hedging instruments \| Foreign exchange contracts
Derivative [Line Items]
Notional amount	4,638,136	3,407,522
Derivative assets - Fair value	59,512	21,624
Derivative liabilities - Fair value	50,397	19,329
Derivatives not designated as hedging instruments \| Interest rate contracts
Derivative [Line Items]
Notional amount	14,139,233	11,695,499
Derivative assets - Fair value	264,603	69,818
Derivative liabilities - Fair value	168,113	69,267
Derivatives not designated as hedging instruments \| Interest rate contracts \| Financial Counterparty
Derivative [Line Items]
Notional amount	7,070,000	5,850,000
Derivatives not designated as hedging instruments \| Interest rate contracts \| LCH
Derivative [Line Items]
Notional amount	2,250,000	1,660,000
Derivative assets - Fair value	317	16,400
Derivative liabilities - Fair value	103,300	16,000
Derivatives not designated as hedging instruments \| Credit contracts
Derivative [Line Items]
Notional amount	211,343	119,320
Derivative assets - Fair value	4	1
Derivative liabilities - Fair value	$ 122	$ 164
Weighted average remaining maturity of outstanding RPAs	2 years 4 months 24 days	6 years 7 months 6 days
Derivatives not designated as hedging instruments \| Credit contracts \| RPA protection sold
Derivative [Line Items]
Maximum exposure of RPAs with protection sold	$ 117	$ 125
Derivatives not designated as hedging instruments \| Equity, Public Companies
Derivative [Line Items]
Number of companies that issued the equity (issuers portion only) \| company	3	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 assets - Fair value	1,542	1,951
Derivative liabilities - Fair value	0	0
Derivatives not designated as hedging instruments \| Commodity contracts
Derivative [Line Items]
Notional amount	0	0
Derivative assets - Fair value	32,978	14,422
Derivative liabilities - Fair value	41,867	23,068
Derivatives not designated as hedging instruments \| Commodity contracts \| Chicago Mercantile Exchange (CME)
Derivative [Line Items]
Derivative assets - Fair value	13,200	10,400
Derivative liabilities - Fair value		582
Derivative assets (liabilities), at fair value, net	$ 3,000	$ 622
Derivatives not designated as hedging instruments \| Commodity contracts \| Chicago Mercantile Exchange (CME) \| Oil
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure \| Boe	2,160	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	3,980	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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Nominal energy measure used to calculate payments on a derivative instrument.

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