Summarized Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2013

Quarterly Financial Information Disclosure [Abstract]

Summarized Quarterly Financial Data (Unaudited)

	Quarter Ended
(Thousands of Dollars)	March 31, 2013		June 30, 2013		Sept. 30, 2013		Dec. 31, 2013
Operating revenues	$	241,415	$	210,175	$	231,060	$	240,388
Operating income	37,401		22,466		40,769		16,545
Net income	19,685		10,544		22,013		7,225

	Quarter Ended
(Thousands of Dollars)	March 31, 2012		June 30, 2012		Sept. 30, 2012		Dec. 31, 2012
Operating revenues	$	223,799	$	194,173	$	226,475	$	217,395
Operating income	28,914		14,348		40,735		15,869
Net income	14,878		5,742		22,200		7,131