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	$	460,072	$	479,796	$	551,623	$	426,509
Operating income	58,415		74,489		122,407		41,498
Net income	25,055		35,362		67,781		31,015

	Quarter Ended
(Thousands of Dollars)	March 31, 2016		June 30, 2016		Sept. 30, 2016		Dec. 31, 2016
Operating revenues	$	390,839	$	440,445	$	554,926	$	464,749
Operating income	53,569		68,386		122,362		62,964
Net income	22,523		32,211		68,346		29,077