Fair Value of Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Assets:
Investments	$ 16.8	$ 14.4
Energy Marketing Contracts Assets, Current	0.1	0.6
Liabilities:
Energy Marketing Contract Liabilities, Current	0.5	0.0
Quoted Prices in Active Markets (Level 1) [Member]
Assets:
Assets, Fair Value Disclosure	15.9	13.3
Quoted Prices in Active Markets (Level 1) [Member] | Investments - Mutual funds and other [Member]
Assets:
Investments	15.9	13.3
Quoted Prices in Active Markets (Level 1) [Member] | Total Investments [Member]
Assets:
Investments	15.9	13.3
Significant Other Observable Inputs (Level 2) [Member]
Assets:
Assets, Fair Value Disclosure	0.1	0.7
Significant Unobservable Inputs (Level 3) [Member]
Assets:
Assets, Fair Value Disclosure	0.9	1.1
Significant Unobservable Inputs (Level 3) [Member] | Investments in guaranteed income fund [Member]
Assets:
Investments	0.9	1.1
Significant Unobservable Inputs (Level 3) [Member] | Total Investments [Member]
Assets:
Investments	0.9	1.1
Recurring [Member]
Assets:
Assets, Fair Value Disclosure	16.9	15.1
Liabilities:
Energy Marketing Contract Liabilities, Current		0.1
Recurring [Member] | Investments in guaranteed income fund [Member]
Assets:
Investments	0.9	1.1
Recurring [Member] | Investments - Mutual funds and other [Member]
Assets:
Investments		13.3
Recurring [Member] | Total Investments [Member]
Assets:
Investments	16.8	14.4
Recurring [Member] | Mark To Market Energy Assets incl. natural gas and swap agreements[Member]
Assets:
Derivative assets	$ 0.1	$ 0.7