CAPITAL CONSTRUCTION FUND - Summary of the activities of the CCF cash and cash equivalents, and investments account (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
CCF Cash and Cash Equivalents:
CCF cash and cash equivalents balance at beginning of period	$ 599.4	$ 518.2
Cash deposits into the CCF	50.0	100.0
Cash paid for purchase of U.S. Treasury debt securities and accrued interest	(449.8)
Proceeds from U.S. Treasury debt securities at maturity	48.1
Interest income deposited into the CCF	18.8	31.1
Repurchase of assigned accounts receivable	53.8
Qualifying withdrawal payments out of the CCF	(89.6)	(49.9)
Total CCF cash and cash equivalents balance at end of period	230.7	599.4
CCF Investments:
Purchase of U.S. Treasury debt securities	448.1
Withdrawals of U.S. Treasury debt securities at maturity	(48.1)
Accretion of investments	11.9
Total CCF investments balance at end of period	411.9
Cash and cash equivalents, and investments account	$ 642.6	$ 599.4