Borrowings and Other Financing Instruments Forward Equity Agreement (Details) - Forward Equity Agreements [Member] - USD ($) $ / shares in Units, shares in Millions, $ in Millions	Mar. 31, 2020	Nov. 04, 2019
Forward Contract Indexed to Issuer's Equity [Line Items]
Common Stock, Value, to be Issued through Forward Equity Agreement		$ 743
Common Stock, Shares to be Issued through forward Equity Agreement		11.8
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		$ 734
Period End Net Cash Settlement, Amount in Cash		$ 19
Period End Net Share Settlement, Amount in Shares		0.3
Common shares, price per share used in forward calculation		$ 62.69
Common stock, interest rate spread used in forward calculation		0.75%
Minimum [Member]
Forward Contract Indexed to Issuer's Equity [Line Items]
Expected settlement price	$ 720
Maximum [Member]
Forward Contract Indexed to Issuer's Equity [Line Items]
Expected settlement price	$ 735