CAPITAL CONSTRUCTION FUND - Summary of the activities of the CCF cash and cash equivalents, and investments account (Details) - USD ($) $ in Millions	1 Months Ended	9 Months Ended
	Feb. 29, 2024	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
CCF Cash and Cash Equivalents:
CCF cash balance at beginning of period		$ 596.7	$ 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)
Interest income deposited into the CCF		15.4	20.8
Qualifying withdrawal payments out of the CCF		(35.8)	(49.9)
Total CCF cash balance at end of period		176.5	589.1
Accrued interest earned on CCF Cash and Investments at end of period		2.2	2.5
Total CCF cash and cash equivalents balance at end of period		178.7	591.6	$ 599.4
CCF Investments:
Purchase of U.S. Treasury debt securities	$ 448.1	448.1
Accretion of investments		8.6
Total CCF investments balance at end of period		456.7
Accrued interest earned on Cash and Cash Equivalents at end of period:		2.2		2.7
Total CCF cash and cash equivalents, and investments balance at end of period		$ 635.4	$ 591.6	$ 599.4
Weighted average life of securities held within the fund		68 days
Capital construction fund, accrued interest on investments	$ 1.7
Debt security investments maturity term	3 years