Quarterly financial data (unaudited) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly financial data (unaudited)
Operating revenues	$ 265,388	$ 349,725	$ 324,349	$ 305,914	$ 279,787	$ 377,396	$ 347,703	$ 319,224	$ 1,245,376	$ 1,324,110	$ 1,390,278
Operating margin	23,909	70,266	69,795	67,554	34,807	61,051	63,919	62,056	231,524	221,833	237,820
Net margin	$ (25,150)	$ 20,104	$ 24,502	$ 22,024	$ (8,821)	$ 23,743	$ 10,878	$ 13,520	$ 41,480	$ 39,320	$ 37,737
Margins for interest ratio	1.14				1.14				1.14	1.14	1.14