Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2016
Debt Instrument [Line Items]
Cash	$ 12,400			$ 8,300
Long-term nonutility investments	$ 4,800			$ 5,200
Nonutility assets representation rate (percentage, less than)	1.00%		1.00%
Net income	$ 26,996	$ 27,405
Utility Segment [Member]
Debt Instrument [Line Items]
Net income	$ 32,300	$ 33,600