CAPITAL CONSTRUCTION FUND - Summary of the activities of the CCF cash and cash equivalents, and investments account (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Feb. 29, 2024	Mar. 31, 2024	Mar. 31, 2023	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			100.0
Cash paid for purchase of U.S. Treasury debt securities and accrued interest		(449.8)
Interest earned on deposits and investments, net		5.1	5.5
Total CCF cash and cash equivalents balance at end of period		154.7	623.7
CCF Investments:
Purchase of U.S. Treasury debt securities	$ 448.1	448.1
Accretion of investments		1.9
Total CCF investments balance at end of period		450.0
Accrued interest earned on Cash and Cash Equivalents, and Investments at end of period:		2.1
Total CCF cash and cash equivalents, and investments balance at end of period		$ 606.8	$ 623.7	$ 599.4
Weighted average life of securities held within the fund		77 days
Capital construction fund, accrued interest on investments	$ 1.7
Debt security investments maturity term	3 years