Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Debt Instrument [Line Items]
Cash	$ 12,900		$ 12,900		$ 1,700
Long-term nonutility investments	5,200		$ 5,200		$ 5,200
Nonutility assets representation rate (percentage, less than)			1.00%		1.00%
Net income	47,256	$ 28,646	$ 104,733	$ 45,590
Utility Segment [Member]
Debt Instrument [Line Items]
Net income	$ 51,900	$ 35,200	$ 122,000	$ 80,200