Income Taxes (Narrative) (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($) generator	Jun. 30, 2018	Jun. 30, 2019 generator	Jun. 30, 2018	Dec. 31, 2018 USD ($)
Income Tax Credits [Line Items]
Effective tax rate (as a percent)	32.60%	12.50%	24.80%	12.10%
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