Summarized Quarterly Financial Data (Unaudited) (Details) - USD ($) $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 464,600		$ 540,100		$ 481,300		$ 447,200		$ 426,500		$ 551,600		$ 479,800		$ 460,100		$ 1,933,200	$ 1,918,000	$ 1,851,000
Operating income	56,000	[1]	111,000	[1]	87,600	[1]	57,100	[1]	43,400	[1]	123,100	[1]	75,200	[1]	59,200	[1]	311,700	300,900	311,300
Net income	$ 40,200		$ 81,500		$ 58,500		$ 33,100		$ 31,000		$ 67,800		$ 35,300		$ 25,100		$ 213,300	$ 159,200	$ 152,200

[1] (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.