Borrowings and Other Financing Instruments, Letters of Credit (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
Jun. 30, 2014	Dec. 31, 2013
Letters of Credit [Abstract]
Terms of letters of credit (in years)	1 year
Letters of credit outstanding under credit facilities	$ 41.0	$ 25.5

An irrevocable undertaking to guarantee payment of a specified financial obligation. Letters of credit issued under the credit facilities.

No definition available.

Name:	sps_LettersOfCreditUnderCreditFacilities
Namespace Prefix:	sps_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant