CAPITAL CONSTRUCTION FUND - Summary of the activities of the CCF cash and cash equivalents, and investments account (Details) - USD ($) $ in Millions	1 Months Ended	6 Months Ended
	Feb. 29, 2024	Jun. 30, 2024	Jun. 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			100.0
Cash paid for purchase of U.S. Treasury debt securities and accrued interest		(449.8)
Interest income deposited into the CCF		11.0	13.1
Repurchase of assigned accounts receivable		35.8
Qualifying withdrawal payments out of the CCF		(35.8)	(49.9)
Total CCF cash balance at end of period		157.9	581.4
Accrued interest earned on CCF Cash and Investments at end of period		2.7	2.5
Total CCF cash and cash equivalents balance at end of period		160.6	583.9	$ 599.4
CCF Investments:
Purchase of U.S. Treasury debt securities	$ 448.1	448.1
Accretion of investments		5.2
Total CCF investments balance at end of period		453.3
Total CCF cash and cash equivalents, and investments balance at end of period		$ 613.9	$ 583.9	$ 599.4
Weighted average life of securities held within the fund		83 days
Capital construction fund, accrued interest on investments	$ 1.7
Debt security investments maturity term	3 years