Electric Plant - Additional Information (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) kV unit facility transmission_line turbine generator mi ft MW	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Segment Reporting Information [Line Items]
Construction work in progress	$ 736,996	$ 541,323
Interest costs capitalized	$ 30,300	13,800	$ 900
Clover [Member]
Segment Reporting Information [Line Items]
Undivided ownership interest	50.00%
Number of units | unit	2
Power facility output | MW	877
Outstanding accounts payable balance	$ 8,200	8,700
Construction work in progress	$ 3,927	16,750
North Anna [Member]
Segment Reporting Information [Line Items]
Undivided ownership interest	11.60%
Number of units | unit	2
Power facility output | MW	1,892
Outstanding accounts payable balance	$ 3,800	3,100
Percentage of costs responsible for	11.60%
Construction work in progress	$ 15,181	32,209
Combustion Turbine Facilities [Member]
Segment Reporting Information [Line Items]
Number of combustion facilities | facility	3
Transmission lines | ft	1,100
Number of Transmission Lines | transmission_line	2
Transmission line capacity | kV	500
Substation capacity | kV	500
Construction work in progress	$ 62	48
Distributed Generation Facilities [Member]
Segment Reporting Information [Line Items]
Number of distributed facilities | facility	6
Other [Member]
Segment Reporting Information [Line Items]
Transmission lines | mi	110
Construction work in progress	$ 1,971	3,616
Wildcat Point [Member]
Segment Reporting Information [Line Items]
Power facility output | MW	1,000
Number of combustion turbines | turbine	2
Number of heat recovery steam generators | generator	2
Number of steam turbine generators | generator	1
Estimated project cost	$ 834,300
Construction work in progress	715,855	$ 488,700
Interest costs capitalized	$ 42,600