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Activity in Company's Allowance for Doubtful Accounts (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Accounts and Other Receivables [Line Items]
Balance at Beginning of Period	$ 1.6	$ 1.9	$ 2.3
Provision	6.0us-gaap_ProvisionForDoubtfulAccounts	4.6us-gaap_ProvisionForDoubtfulAccounts	3.6us-gaap_ProvisionForDoubtfulAccounts
Recoveries	0.6	0.4	0.6
Accounts Written Off	6.4	5.3	4.6
Balance at End of Period	1.8	1.6	1.9
Electric
Accounts and Other Receivables [Line Items]
Balance at Beginning of Period	1.3	1.1	1.7
Provision	2.9	2.6	1.4
Recoveries	0.3	0.2	0.3
Accounts Written Off	3.2	2.6	2.3
Balance at End of Period	1.3	1.3	1.1
Gas
Accounts and Other Receivables [Line Items]
Balance at Beginning of Period	0.2	0.7	0.5
Provision	3.1	2.0	2.2
Recoveries	0.3	0.2	0.3
Accounts Written Off	3.2	2.7	2.3
Balance at End of Period	0.4	0.2	0.7
Other
Accounts and Other Receivables [Line Items]
Balance at Beginning of Period				0.1
Balance at End of Period	$ 0.1	$ 0.1	$ 0.1	$ 0.1