Fair Value of Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Assets:
Investments	$ 14.4	$ 12.3
Fair Value, Inputs, Level 1 [Member]
Assets:
Assets, Fair Value Disclosure	13.3	10.8
Fair Value, Inputs, Level 1 [Member] | Mark-to-market energy liabilities
Liabilities:
Mark-to-market energy liabilities	0.0	0.0
Significant Other Observable Inputs (Level 2)
Assets:
Assets, Fair Value Disclosure	0.7	1.0
Significant Other Observable Inputs (Level 2) | Mark-to-market energy liabilities
Liabilities:
Mark-to-market energy liabilities	0.1	1.3
Significant Unobservable Inputs (Level 3)
Assets:
Assets, Fair Value Disclosure	1.1	1.5
Significant Unobservable Inputs (Level 3) | Mark-to-market energy liabilities
Liabilities:
Mark-to-market energy liabilities	0.0	0.0
Equity Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Assets:
Investments	0.0	0.0
Equity Securities [Member] | Significant Other Observable Inputs (Level 2)
Assets:
Investments	0.0	0.0
Equity Securities [Member] | Significant Unobservable Inputs (Level 3)
Assets:
Investments	0.0	0.0
Guaranteed Income Fund [Member] | Fair Value, Inputs, Level 1 [Member]
Assets:
Investments	0.0	0.0
Guaranteed Income Fund [Member] | Significant Other Observable Inputs (Level 2)
Assets:
Investments	0.0	0.0
Guaranteed Income Fund [Member] | Significant Unobservable Inputs (Level 3)
Assets:
Investments	1.1	1.5
Other Investments [Member] | Fair Value, Inputs, Level 1 [Member]
Assets:
Investments	13.3	10.8
Other Investments [Member] | Significant Other Observable Inputs (Level 2)
Assets:
Investments	0.0	0.0
Other Investments [Member] | Significant Unobservable Inputs (Level 3)
Assets:
Investments	0.0	0.0
Investments [Member] | Fair Value, Inputs, Level 1 [Member]
Assets:
Investments	13.3	10.8
Investments [Member] | Significant Other Observable Inputs (Level 2)
Assets:
Investments	0.0	0.0
Investments [Member] | Significant Unobservable Inputs (Level 3)
Assets:
Investments	1.1	1.5
Mark-to-market energy assets, including put option | Fair Value, Inputs, Level 1 [Member]
Assets:
Mark-to-market energy assets, including put option	0.0	0.0
Mark-to-market energy assets, including put option | Significant Other Observable Inputs (Level 2)
Assets:
Mark-to-market energy assets, including put option	0.7	1.0
Mark-to-market energy assets, including put option | Significant Unobservable Inputs (Level 3)
Assets:
Mark-to-market energy assets, including put option	0.0	0.0
Fair Value
Assets:
Assets, Fair Value Disclosure	15.1	13.3
Fair Value | Mark-to-market energy liabilities
Liabilities:
Mark-to-market energy liabilities	0.1	1.3
Fair Value | Equity Securities [Member]
Assets:
Investments	0.0	0.0
Fair Value | Guaranteed Income Fund [Member]
Assets:
Investments	1.1	1.5
Fair Value | Other Investments [Member]
Assets:
Investments	13.3	10.8
Fair Value | Investments [Member]
Assets:
Investments	14.4	12.3
Fair Value | Mark-to-market energy assets, including put option
Assets:
Mark-to-market energy assets, including put option	$ 0.7	$ 1.0