Quarterly financial data (unaudited): (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Quarterly financial data (unaudited):
Operating revenues	$ 297,642	$ 368,664	$ 343,741	$ 339,778	$ 315,475	$ 369,405	$ 355,983	$ 367,300	$ 1,349,825	$ 1,408,163	$ 1,245,376
Operating margin	56,417	69,287	63,494	68,260	49,255	70,685	69,600	69,857	257,458	259,397	231,524
Net margin	$ 6,212	$ 15,908	$ 10,852	$ 15,369	$ (4,237)	$ 14,453	$ 17,196	$ 19,223	$ 48,341	$ 46,635	$ 41,480
Margins for interest ratio	1.14				1.14				1.14	1.14	1.14