Debt (Revolving Credit Facility) (Details)	Jan. 17, 2017 USD ($)	Dec. 31, 2018
Revolving credit facility
Line of Credit Facility [Line Items]
Capitalization ratio		0.6
Line of Credit | Revolving credit facility
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 2,500,000,000
Line of credit facility, conditional increase in maximum borrowing	500,000,000
Line of Credit | JPMorgan Chase Bank | Letter of credit
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,500,000,000
LIBOR and EURIBOR rate | Minimum | Revolving credit facility
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	0.82%
LIBOR and EURIBOR rate | Maximum | Revolving credit facility
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	1.30%
Base rate | Minimum | Revolving credit facility
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	0.00%
Base rate | Maximum | Revolving credit facility
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	0.30%
Federal funds rate and overnight bank funding rate spread | Revolving credit facility
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	0.50%
London Interbank Offered Rate (LIBOR) | Revolving credit facility
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	1.00%