Derivatives (Narrative) (Details) MMBTU in Thousands, Boe in Thousands, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($) MMBTU Boe company	Dec. 31, 2018 USD ($) company	Dec. 31, 2018 USD ($) MMBTU company	Dec. 31, 2018 USD ($) Boe company
Derivative [Line Items]
Derivative liabilities - Fair value	$ 172,923	$ 118,305	$ 118,305	$ 118,305
Derivative assets - Fair value	$ 242,876	107,816	$ 107,816	107,816
Natural gas
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	30,775		14,722
Credit-risk-related contingent features
Derivative [Line Items]
Derivative asset	$ 1,000	2,800	$ 2,800	2,800
Derivative liability	53,900	14,200	14,200	14,200
Aggregate fair value of derivative instruments in net liability position	52,900	11,400	11,400	11,400
Collateral posted	52,700	9,400	9,400	9,400
Derivative instruments designated as hedging instruments
Derivative [Line Items]
Notional amount	191,033	126,056	126,056	126,056
Derivative liabilities - Fair value	3,019	6,477	6,477	6,477
Derivative assets - Fair value	52	0	0	0
Derivative instruments designated as hedging instruments | Interest rate contracts
Derivative [Line Items]
Notional amount	31,026	35,811	35,811	35,811
Derivative liabilities - Fair value	3,019	5,866	5,866	5,866
Derivative assets - Fair value	0	0	0	0
Derivative instruments designated as hedging instruments | Foreign exchange contracts
Derivative [Line Items]
Notional amount	160,007	90,245	90,245	90,245
Derivative liabilities - Fair value	0	611	611	611
Derivative assets - Fair value	52	0	0	0
Derivatives not designated as hedging instruments
Derivative [Line Items]
Notional amount	16,986,784	15,222,341	15,222,341	15,222,341
Derivative liabilities - Fair value	169,904	111,828	111,828	111,828
Derivative assets - Fair value	242,824	107,816	107,816	107,816
Derivatives not designated as hedging instruments | Interest rate contracts
Derivative [Line Items]
Notional amount	13,469,821	11,695,499	11,695,499	11,695,499
Derivative liabilities - Fair value	121,352	69,267	69,267	69,267
Derivative assets - Fair value	190,388	69,818	69,818	69,818
Derivatives not designated as hedging instruments | Interest rate contracts | LCH
Derivative [Line Items]
Notional amount	2,080,000	1,660,000	1,660,000	1,660,000
Derivative asset	1,500	16,400	16,400	16,400
Derivative liability	73,500	16,000	16,000	16,000
Derivatives not designated as hedging instruments | Interest rate contracts | Financial Counterparty
Derivative [Line Items]
Notional amount	6,740,000	5,850,000	5,850,000	5,850,000
Derivatives not designated as hedging instruments | Foreign exchange contracts
Derivative [Line Items]
Notional amount	3,425,107	3,407,522	3,407,522	3,407,522
Derivative liabilities - Fair value	22,528	19,329	19,329	19,329
Derivative assets - Fair value	27,797	21,624	21,624	21,624
Derivatives not designated as hedging instruments | Credit contracts
Derivative [Line Items]
Notional amount	91,856	119,320	119,320	119,320
Derivative liabilities - Fair value	$ 118	$ 164	164	164
Weighted average remaining maturity of outstanding RPAs	4 years 7 months 6 days	6 years 7 months 6 days
Derivative assets - Fair value	$ 3	$ 1	1	1
Derivatives not designated as hedging instruments | Credit contracts | RPA protection sold
Derivative [Line Items]
Maximum exposure of RPAs with protection sold	$ 26	$ 125	$ 125	$ 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	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	18	18
Derivatives not designated as hedging instruments | Equity contracts
Derivative [Line Items]
Notional amount	$ 0	$ 0	$ 0	$ 0
Derivative liabilities - Fair value	0	0	0	0
Derivative assets - Fair value	1,985	1,951	1,951	1,951
Derivatives not designated as hedging instruments | Commodity contracts
Derivative [Line Items]
Notional amount	0	0	0	0
Derivative liabilities - Fair value	25,906	23,068	23,068	23,068
Derivative assets - Fair value	22,651	14,422	14,422	14,422
Derivatives not designated as hedging instruments | Commodity contracts | Chicago Mercantile Exchange (CME)
Derivative [Line Items]
Derivative asset	6,200	10,400	10,400	10,400
Derivative liability	13	(582)	(582)	(582)
Derivative assets (liabilities), at fair value, net	$ 1,700	$ 622	$ 622	$ 622
Derivative, nonmonetary notional amount, energy measure			6,290	778
Derivatives not designated as hedging instruments | Commodity contracts | Chicago Mercantile Exchange (CME) | Oil
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | Boe	1,697
Derivatives not designated as hedging instruments | Commodity contracts | Chicago Mercantile Exchange (CME) | Natural gas
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	5,150
Maximum | Derivatives not designated as hedging instruments | Foreign exchange contracts
Derivative [Line Items]
Original maturity (in years)	1 year	1 year