Summarized Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2016

Quarterly Financial Information Disclosure [Abstract]

Summarized Quarterly Financial Data (Unaudited)

	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

	Quarter Ended
(Thousands of Dollars)	March 31, 2015		June 30, 2015		Sept. 30, 2015		Dec. 31, 2015
Operating revenues	$	273,960	$	216,813	$	236,161	$	229,607
Operating income	39,549		25,069		47,532		27,809
Net income	22,267		12,512		26,232		13,625