Quarterly Financial information Data (Unaudited) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Selected Quarterly Financial Information [Abstract]
Operating Revenue	$ 67,432	$ 72,111	$ 68,038	$ 70,283	$ 68,751	$ 67,729	$ 60,741	$ 71,267	$ 277,864	$ 268,488	$ 254,027
Operating Income	21,825	23,456	21,143	21,490	18,779	19,007	13,782	17,546	87,914	69,114	62,344
Net income	$ 11,937	$ 12,287	$ 10,547	$ 10,403	$ 8,773	$ 9,916	$ 6,230	$ 8,643	$ 45,174	$ 33,562	$ 30,173