Income Taxes (Narrative) (Details) $ in Millions	3 Months Ended			9 Months Ended		60 Months Ended
	Sep. 30, 2019 USD ($) generator	Jun. 30, 2019 USD ($)	Sep. 30, 2018	Sep. 30, 2019 generator	Sep. 30, 2018	Dec. 31, 2018 USD ($)
Income Tax Credits [Line Items]
Effective tax rate (as a percent)	16.90%		16.40%	22.20%	13.50%
Reversal of state income taxes payable	$ 6.4
DC Solar Tax Credit Investments
Income Tax Credits [Line Items]
Reversal of certain previously claimed tax credits		$ 30.1
Investment tax credits						$ 53.9
Deferred tax liability						$ 5.7
Tax basis reduction (as a percent)						50.00%
Tax credit claimed, net of tax basis reduction						$ 48.2
Amount reversed of previously claimed tax credit		33.6
Amount reversed of deferred tax liability		$ 3.5
Number of mobile service generators identified | generator	0			0