CAPITAL CONSTRUCTION FUND - Summary of the activities within the CCF cash and cash equivalents, and investments account (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Feb. 29, 2024	Mar. 31, 2025	Mar. 31, 2024	Dec. 31, 2024
CCF Cash and Cash Equivalents:
CCF cash and cash equivalents balance at beginning of period		$ 230.7	$ 599.4
Cash withdrawal for the purchase of U.S. Treasury debt securities and accrued interest			(449.8)
Proceeds from U.S. Treasury debt securities at maturity		20.3
Interest income on cash and cash equivalents, and CCF investments		4.1	7.2
Repurchase of assigned accounts receivable		100.7
Qualifying withdrawal payments out of the CCF		(65.0)
Total CCF cash and cash equivalents balance at end of period		290.8	156.8
CCF Investments:
CCF investments balance at beginning of period		411.9
Purchase of U.S. Treasury debt securities	$ 448.1		448.1
Withdrawals of U.S. Treasury debt securities at maturity		(20.3)
Accretion of investments		3.0	1.9
Total CCF investments balance at end of period		394.6	450.0
Total CCF cash and cash equivalents, and investments balance at end of period		$ 685.4	$ 606.8	$ 642.6
Weighted average life of securities held within the fund		89 days
Capital construction fund, accrued interest on investments	$ 1.7
Debt security investments maturity term		2 years