Activity in Company's Allowance for Doubtful Accounts (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at Beginning of Period		$ 2.6	$ 3.3	$ 3.3
Provision		6.4	6.7	5.6
Recoveries		0.6	0.9	0.8
Accounts Written Off		7.2	7.6	6.5
Regulatory Defferals	[1]	0.0	(0.7)	0.1
Balance at End of Period		2.4	2.6	3.3
Electric
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at Beginning of Period		1.6	2.0	1.6
Provision		3.6	4.2	3.3
Recoveries		0.2	0.3	0.4
Accounts Written Off		3.9	4.4	3.4
Regulatory Defferals	[1]	0.0	(0.5)	0.1
Balance at End of Period		1.5	1.6	2.0
Gas
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at Beginning of Period		1.0	1.3	1.7
Provision		2.8	2.5	2.3
Recoveries		0.4	0.6	0.4
Accounts Written Off		3.3	3.2	3.1
Regulatory Defferals	[1]	0.0	(0.2)	0.0
Balance at End of Period		$ 0.9	$ 1.0	$ 1.3

[1] In 2021, the Company recorded higher than normal expected bad debt expense due to the coronavirus pandemic. The incremental bad debt expense amounts were previously deferred as regulatory assets based on certain regulatory proceedings and management’s view that such amounts were probable of recovery. Based on actual billing and collections experience, the Company has not deferred any incremental bad debt expense as a regulatory asset as of December 31, 2023 and December 31, 2022.