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	$	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

	Quarter Ended
(Thousands of Dollars)	March 31, 2014		June 30, 2014		Sept. 30, 2014		Dec. 31, 2014
Operating revenues	$	448,400	$	492,536	$	552,779	$	443,655
Operating income	42,576		59,000		118,769		45,779
Net income	18,735		28,035		66,937		16,145