Summarized Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2015

Quarterly Financial Information Disclosure [Abstract]

Summarized Quarterly Financial Data (Unaudited)

	Quarter Ended
(Thousands of Dollars)	March 31, 2015		June 30, 2015		Sept. 30, 2015		Dec. 31, 2015
Operating revenues	$	1,135,450	$	952,521	$	1,044,704	$	1,030,838
Operating income	215,400		195,176		315,174		173,951
Net income	110,966		98,500		173,081		84,255

	Quarter Ended
(Thousands of Dollars)	March 31, 2014		June 30, 2014		Sept. 30, 2014		Dec. 31, 2014
Operating revenues	$	1,203,543	$	993,704	$	1,049,111	$	1,136,791
Operating income	208,437		163,437		261,073		169,423
Net income	118,403		89,792		154,159		92,834