Allowance for Doubtful Accounts - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Recoveries	$ 0.6	$ 0.8	$ 0.6
Provision for Bad Debt	5.5	4.9	6.0
Allowance for doubtful accounts	3.3	1.0	1.3	$ 1.6
Unbilled Revenues
Financing Receivable, Impaired [Line Items]
Allowance for doubtful accounts	0.2
Accounts Receivable
Financing Receivable, Impaired [Line Items]
Allowance for doubtful accounts	3.1	1.0
Regulatory Assets Harship Accounts
Financing Receivable, Impaired [Line Items]
Recoveries	6.8	5.6
Energy Commodity
Financing Receivable, Impaired [Line Items]
Provision for Bad Debt	$ 1.6	$ 2.3	$ 2.6