Quarterly financial data (unaudited): (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Quarterly financial data (unaudited):
Operating revenues	$ 348,714	$ 431,013	$ 379,343	$ 348,161	$ 297,642	$ 368,664	$ 343,741	$ 339,778	$ 1,507,231	$ 1,349,825	$ 1,408,163
Operating margin	39,494	70,929	74,148	71,093	56,417	69,287	63,494	68,260	255,664	257,458	259,397
Net Margin	$ (12,160)	$ 18,630	$ 23,277	$ 20,598	$ 6,212	$ 15,908	$ 10,852	$ 15,369	$ 50,345	$ 48,341	$ 46,635
Margins for interest ratio	1.14				1.14				1.14	1.14	1.14