FAIR VALUE MEASUREMENT -Fair Value Hierarchy for Assets and Liabilities on a Recurring Basis (Details) - Fair Value, Recurring - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Assets:
Note receivable - building sale		$ 6.2
Total assets	$ 1,578.3	116.5
US And UK Treasury securities
Assets:
Cash and cash equivalents	1,294.1
Money market funds
Assets:
Cash and cash equivalents	248.1
US Treasury Securities
Assets:
Investments	0.3	70.0
Mutual funds
Assets:
Investments	28.5	23.8
Money market funds
Assets:
Investments	7.3	16.5
Level 1
Assets:
Note receivable - building sale		0.0
Total assets	1,578.3	110.3
Level 1 | US And UK Treasury securities
Assets:
Cash and cash equivalents	1,294.1
Level 1 | Money market funds
Assets:
Cash and cash equivalents	248.1
Level 1 | US Treasury Securities
Assets:
Investments	0.3	70.0
Level 1 | Mutual funds
Assets:
Investments	28.5	23.8
Level 1 | Money market funds
Assets:
Investments	7.3	16.5
Level 2
Assets:
Note receivable - building sale		0.0
Total assets	0.0	0.0
Level 2 | US And UK Treasury securities
Assets:
Cash and cash equivalents	0.0
Level 2 | Money market funds
Assets:
Cash and cash equivalents	0.0
Level 2 | US Treasury Securities
Assets:
Investments	0.0	0.0
Level 2 | Mutual funds
Assets:
Investments	0.0	0.0
Level 2 | Money market funds
Assets:
Investments	0.0	0.0
Level 3
Assets:
Note receivable - building sale		6.2
Total assets	0.0	6.2
Level 3 | US And UK Treasury securities
Assets:
Cash and cash equivalents	0.0
Level 3 | Money market funds
Assets:
Cash and cash equivalents	0.0
Level 3 | US Treasury Securities
Assets:
Investments	0.0	0.0
Level 3 | Mutual funds
Assets:
Investments	0.0	0.0
Level 3 | Money market funds
Assets:
Investments	$ 0.0	$ 0.0