Activity in Company's Allowance for Doubtful Accounts (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at Beginning of Period		$ 1.0	$ 1.3	$ 1.6
Provision		5.5	4.9	6.0
Recoveries		0.6	0.8	0.6
Accounts Written Off		4.4	6.0	6.9
Regulatory Defferals	[1]	0.6
Balance at End of Period		3.3	1.0	1.3
Electric
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at Beginning of Period		0.6	0.5	0.9
Provision		2.9	3.0	3.2
Recoveries		0.3	0.3	0.3
Accounts Written Off		2.6	3.2	3.9
Regulatory Defferals	[1]	0.4
Balance at End of Period		1.6	0.6	0.5
Gas
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at Beginning of Period		0.4	0.8	0.6
Provision		2.6	1.9	2.9
Recoveries		0.3	0.5	0.3
Accounts Written Off		1.8	2.8	3.0
Regulatory Defferals	[1]	0.2
Balance at End of Period		$ 1.7	$ 0.4	0.8
Others
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at Beginning of Period				0.1
Provision				$ (0.1)

[1] The Company has incurred greater than normal bad debt expense due to the coronavirus pandemic. Incremental bad debt expense amounts have been deferred as regulatory assets based on certain regulatory proceedings and management’s belief that such amounts are probable of recovery (See the “Financial Effects of COVID-19 Pandemic” section in Note 8 (Commitments and Contingencies). The Company will track the collection of receivables and to the extent incremental bad debt amounts are collected in the future, such amounts will reduce the regulatory assets recorded.