Borrowings and Other Financing Instruments Forward Equity Agreements (Details) - Forward Equity Agreements [Member] - USD ($) $ / shares in Units, shares in Millions, $ in Millions	Dec. 31, 2019	Nov. 01, 2019	Aug. 29, 2019	Nov. 30, 2018
Forward Contract Indexed to Issuer's Equity [Line Items]
Common stock, value, to be issued through Forward Equity Agreement		$ 743		$ 459
Common stock, shares to be issued through Forward Equity Agreement		11.8		9.4
Common Stock, Shares Issued through Forward Equity Agreement			9.4
Common Stock, Value Issued through Forward Equity Agreement			$ 453
Common stock, initial shares in an agreement		10.3
Common stock, additional shares in an agreement		1.5
Period end settlement price, in shares	11.8
Period end settlement price, in cash	$ 739
Period end net cash settlement price	$ 6
Period end net share settlement price	0.1
Common shares, price per share used in forward calculation		$ 62.69
Common stock, interest rate spread used in forward calculation		0.75%
Minimum
Forward Contract Indexed to Issuer's Equity [Line Items]
Expected settlement price	$ 730
Maximum
Forward Contract Indexed to Issuer's Equity [Line Items]
Expected settlement price	$ 740