Quarterly financial data (unaudited): (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly financial data (unaudited)
Operating revenues	$ 315,475	$ 369,405	$ 355,983	$ 367,300	$ 265,388	$ 349,725	$ 324,349	$ 305,914	$ 1,408,163	$ 1,245,376	$ 1,324,110
Operating margin	49,255	70,685	69,600	69,857	23,909	70,266	69,795	67,554	259,397	231,524	221,833
Net margin	$ (4,237)	$ 14,453	$ 17,196	$ 19,223	$ (25,150)	$ 20,104	$ 24,502	$ 22,024	$ 46,635	$ 41,480	$ 39,320
Margins for interest ratio	1.14				1.14				1.14	1.14	1.14