Fair Value of Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Assets:
Investments	$ 17.2	$ 14.4
Fair Value, Inputs, Level 1 [Member]
Assets:
Assets, Fair Value Disclosure	16.3	13.3
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	0.0	0.7
Significant Other Observable Inputs (Level 2) | Mark-to-market energy liabilities
Liabilities:
Mark-to-market energy liabilities	1.4	0.1
Significant Unobservable Inputs (Level 3)
Assets:
Assets, Fair Value Disclosure	0.9	1.1
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	0.9	1.1
Other Investments [Member] | Fair Value, Inputs, Level 1 [Member]
Assets:
Investments	16.3	13.3
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
Investments [Member] | Significant Other Observable Inputs (Level 2)
Assets:
Investments		0.0
Investments [Member] | Significant Unobservable Inputs (Level 3)
Assets:
Investments		1.1
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
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
Mark-to-market energy assets, including put option | Significant Unobservable Inputs (Level 3)
Assets:
Mark-to-market energy assets, including put option		0.0
Fair Value
Assets:
Assets, Fair Value Disclosure	17.2	15.1
Fair Value | Mark-to-market energy liabilities
Liabilities:
Mark-to-market energy liabilities	1.4	0.1
Fair Value | Equity Securities [Member]
Assets:
Investments	0.0	0.0
Fair Value | Guaranteed Income Fund [Member]
Assets:
Investments	0.9	1.1
Fair Value | Other Investments [Member]
Assets:
Investments	$ 16.3	13.3
Fair Value | Investments [Member]
Assets:
Investments		14.4
Fair Value | Mark-to-market energy assets, including put option
Assets:
Mark-to-market energy assets, including put option		$ 0.7