Quarterly financial data (unaudited) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly financial data (unaudited)
Operating revenues	$ 279,787	$ 377,396	$ 347,703	$ 319,224	$ 308,171	$ 432,530	$ 379,803	$ 269,774	$ 1,324,110	$ 1,390,278	$ 1,294,145
Operating margin	34,807	61,051	63,919	62,056	43,299	64,884	63,702	65,935	221,833	237,820	239,249
Net Margin	$ (8,821)	$ 23,743	$ 10,878	$ 13,520	$ (1,499)	$ 10,408	$ 12,718	$ 16,110	$ 39,320	$ 37,737	$ 33,733
Margins for interest ratio	1.14				1.14				1.14	1.14	1.14