Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Debt Instrument [Line Items]
Cash	$ 10,300		$ 10,300		$ 5,900
Long-term nonutility investments	5,100		$ 5,100		$ 5,100
Nonutility assets representation rate (percentage, less than)			1.00%		1.00%
Net income	28,646	$ 28,471	$ 45,590	$ 63,840
Utility Segment [Member]
Debt Instrument [Line Items]
Net income	$ 35,200	$ 36,300	$ 80,200	$ 87,500