Leases (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Operating Leases after Adoption of ASC Topic 842
Noncurrent operating and finance lease liabilities	$ 1,163	$ 1,549
Mankato Energy Center [Member] \| PPAs \| NSP Minnesota [Member]
Operating Leases after Adoption of ASC Topic 842
Noncurrent operating and finance lease liabilities		$ 400

Present value of lessee's discounted obligation for lease payments from operating lease, classified as noncurrent.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977

Name:	us-gaap_OperatingLeaseLiabilityNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant