Borrowings and Other Financing Instruments Forward Equity Agreement (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Nov. 04, 2019
Forward Contract Indexed to Issuer's Equity [Line Items]
Proceeds from issuance of common stock	$ 5	$ 457
Forward Equity Agreements [Member]
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 shares, price per share used in forward calculation			$ 62.69
Common stock, interest rate spread used in forward calculation			0.75%
Period End Settlement Price, In Shares			11.8
Period End Settlement Price, in Cash			$ 722
Period End Net Cash Settlement Price			$ 92
Period End Net Share Settlement Price			1.3
Common stock, initial shares in an agreement			10.3
Common stock, additional shares in an agreement			1.5
Forward Equity Agreements [Member] | Maximum [Member]
Forward Contract Indexed to Issuer's Equity [Line Items]
Expected settlement price	720
Dividend Reinvestment Program [Member]
Forward Contract Indexed to Issuer's Equity [Line Items]
Proceeds from issuance of common stock	$ 30	$ 29