Basis of Presentation and Significant Accounting Policies - Investment Tax Credits (ITC) (Additional Information) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2016
Tax Credit Carryforward [Line Items]
Benefit of ITC	$ 46.5
ITC recapture period	5 years
Minimum
Tax Credit Carryforward [Line Items]
Incentives received by the Company		1.00%
Maximum
Tax Credit Carryforward [Line Items]
Incentives received by the Company		10.00%
2017
Tax Credit Carryforward [Line Items]
Benefit of ITC	$ 44.9