Investment Securities - Investment Gains and Losses For Level 3 (Details) - Other long-term investments - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Investment securities
Net unrealized investment gains (losses)	$ 195.2	$ (43.8)	$ 172.7
Level 3
Investment securities
Net unrealized investment gains (losses)	$ 133.9	$ 56.5	$ 98.9

Amount of unrealized gain (loss) on investment.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (b)
-SubTopic 10
-Topic 230
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482740/230-10-45-28

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