Summarized Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2012

Summarized Quarterly Financial Data (Unaudited) [Abstract]

Summarized Quarterly Financial Data (Unaudited)

	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

	Quarter Ended
(Thousands of Dollars)	March 31, 2011			June 30, 2011			Sept. 30, 2011			Dec. 31, 2011
Operating revenues		$	238,108		$	198,850		$	224,468		$	214,364
Operating income			29,897			20,115			39,032			18,464
Net income			14,643			8,478			19,973			7,912