Summarized Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Summarized Quarterly Financial Data (Unaudited)

Summarized Quarterly Financial Data (Unaudited)

	Quarter Ended
(Millions of Dollars)	March 31, 2018		June 30, 2018		Sept. 30, 2018		Dec. 31, 2018
Operating revenues	$	447.2	$	481.3	$	540.1	$	464.6
Operating income	57.1		87.6		111.0		56.0
Net income	33.1		58.5		81.5		40.2

	Quarter Ended
(Millions of Dollars)	March 31, 2017		June 30, 2017		Sept. 30, 2017		Dec. 31, 2017
Operating revenues	$	460.1	$	479.8	$	551.6	$	426.5
Operating income (a)	59.2		75.2		123.1		43.4
Net income	25.1		35.3		67.8		31.0

^(a)  In 2018, SPS implemented ASU No. 2017-07 related to net periodic benefit cost, which resulted in retrospective reclassification of pension costs from O&M expense to other income.