Summarized Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2015

Quarterly Financial Information Disclosure [Abstract]

Summarized Quarterly Financial Data (Unaudited)

Summarized Quarterly Financial Data (Unaudited)

	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

	Quarter Ended
(Thousands of Dollars)	March 31, 2014		June 30, 2014		Sept. 30, 2014		Dec. 31, 2014
Operating revenues	$	285,142	$	228,114	$	231,046	$	256,160
Operating income	42,571		23,730		37,540		27,787
Net income	24,235		12,022		20,030		14,355