Activity in Company's Allowance for Doubtful Accounts (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at Beginning of Period	$ 1.1	$ 1.2	$ 1.8
Provision	3.7	4.6	5.3
Recoveries	0.6	0.6	0.7
Accounts Written Off	3.8	5.3	6.6
Balance at End of Period	1.6	1.1	1.2
Electric
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at Beginning of Period	0.8	0.6	1.3
Provision	1.8	2.9	2.5
Recoveries	0.3	0.3	0.3
Accounts Written Off	2.0	3.0	3.5
Balance at End of Period	0.9	0.8	0.6
Gas
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at Beginning of Period	0.2	0.5	0.4
Provision	1.9	1.7	2.8
Recoveries	0.3	0.3	0.4
Accounts Written Off	1.8	2.3	3.1
Balance at End of Period	0.6	0.2	0.5
Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at Beginning of Period	0.1	0.1	0.1
Balance at End of Period	$ 0.1	$ 0.1	$ 0.1