Wholesale Power Contracts (Schedule Of Revenues From Member Distribution Cooperatives) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Related Party Transaction [Line Items]
Member distribution revenue	$ 847.1	$ 968.9	$ 908.0
Rappahannock Electric Cooperative [Member]
Related Party Transaction [Line Items]
Member distribution revenue	271.2	334.2	311.7
Shenandoah Valley Electric Cooperative [Member]
Related Party Transaction [Line Items]
Member distribution revenue	164.5	181.0	172.1
Delaware Electric Cooperative, Inc. [Member]
Related Party Transaction [Line Items]
Member distribution revenue	105.9	114.0	106.8
Choptank Electric Cooperative, Inc. [Member]
Related Party Transaction [Line Items]
Member distribution revenue	77.2	83.8	80.2
Southside Electric Cooperative [Member]
Related Party Transaction [Line Items]
Member distribution revenue	67.9	76.5	70.2
A&N Electric Cooperative [Member]
Related Party Transaction [Line Items]
Member distribution revenue	51.1	55.5	53.0
Mecklenburg Electric Cooperative [Member]
Related Party Transaction [Line Items]
Member distribution revenue	41.2	47.4	43.8
Prince George Electric Cooperative [Member]
Related Party Transaction [Line Items]
Member distribution revenue	22.5	25.4	23.5
Northern Neck Electric Cooperative [Member]
Related Party Transaction [Line Items]
Member distribution revenue	21.3	23.4	21.3
Community Electric Cooperative [Member]
Related Party Transaction [Line Items]
Member distribution revenue	14.4	16.6	15.3
BARC Electric Cooperative [Member]
Related Party Transaction [Line Items]
Member distribution revenue	$ 9.9	$ 11.1	$ 10.1