Summarized Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2016

Quarterly Financial Information Disclosure [Abstract]

Summarized Quarterly Financial Data (Unaudited)

Summarized Quarterly Financial Data (Unaudited)

	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

	Quarter Ended
(Thousands of Dollars)	March 31, 2015		June 30, 2015		Sept. 30, 2015		Dec. 31, 2015
Operating revenues	$	423,829	$	422,985	$	530,752	$	409,652
Operating income	49,759		53,132		117,076		54,498
Net income	20,247		22,576		61,815		22,625