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	$	1,057,841	$	909,852	$	1,059,177	$	1,020,926
Operating income	223,190		180,629		315,605		170,197
Net income	115,874		87,344		173,607		86,666

	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