Summarized Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Summarized Quarterly Financial Data (Unaudited)

	Quarter Ended
(Thousands of Dollars)	March 31, 2017		June 30, 2017		Sept. 30, 2017		Dec. 31, 2017
Operating revenues	$	264,931	$	230,026	$	247,511	$	262,983
Operating income	42,775		29,067		38,392		37,994
Net income	22,419		14,241		22,325		20,431

	Quarter Ended
(Thousands of Dollars)	March 31, 2016		June 30, 2016		Sept. 30, 2016		Dec. 31, 2016
Operating revenues	$	254,850	$	219,173	$	246,144	$	237,066
Operating income	35,448		27,778		46,342		30,360
Net income	17,631		12,625		24,221		14,658