Income Tax (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Mar. 31, 2015	Jun. 30, 2017	Jun. 30, 2016
Income Tax Disclosure [Abstract]
Income taxes at statutory rates (federal and state)	$ 1,725	$ 1,351		$ 28,325	$ 25,959
Increase (decrease):
Differences required to be flowed-through by regulatory commissions	66	65		1,584	1,583
Other, net	(122)	(34)		(1,317)	(774)
Total provision for income taxes	$ 1,669	$ 1,382		$ 28,592	$ 26,768
Effective tax rate	37.90%	40.60%		39.90%	40.90%