Fair Value of Financial Assets and Liabilities, Financial Impact of Qualifying Cash Flow Hedges (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financial Impact of Qualifying Cash Flow Hedges on Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated other comprehensive loss related to cash flow hedges at Jan. 1		$ (58)	$ (51)	$ (55)
After-tax net unrealized losses related to derivatives accounted for as hedges		(5)	0	0
After-tax net realized losses on derivative transactions reclassified into earnings		3	3	4
Adoption of ASU. 2018-02 (a)	[1]	0	(10)	0
Accumulated other comprehensive loss related to cash flow hedges at Dec. 31		$ (60)	$ (58)	$ (51)

[1] In 2017, Xcel Energy implemented ASU No. 2018-02 related to TCJA, which resulted in reclassification of certain credit balances within net accumulated other comprehensive loss to retained earnings.