Basis of Presentation and Significant Accounting Policies (Details) $ in Millions	12 Months Ended
	Dec. 31, 2025 USD ($) segment	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Sep. 02, 2025	Jan. 01, 2021 USD ($)
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of reportable segments | segment	4
Percent of shares outstanding	12.30%
Other long-term investments	$ 2,958.5	$ 2,150.2
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	Other assets	Other assets
Operating lease, right-of-use asset	$ 51.2	$ 32.6
Operating Lease, Liability, Statement of Financial Position [Extensible List]	Other liabilities	Other liabilities
Lease liabilities	$ 56.2	$ 35.5
Net unrealized gains (losses) from foreign currency translation	0.8	(1.7)
Change in fair value of Ark’s contingent consideration	173.0	61.3	$ 48.7
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Other long-term investments	689.7	547.8
Contingent consideration	328.3	155.3			$ 22.5
Change in fair value of Ark’s contingent consideration	173.0	61.3	$ 48.7
Level 2
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Variance threshold for outliers	$ 0.5
Variance threshold for outliers (as a percent)	5.00%
Level 2 | Fair value measured on a recurring basis | Other long-term investments
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Other long-term investments	$ 33.7	23.5
Level 3 | Fair value measured on a recurring basis | Other long-term investments
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Other long-term investments	1,814.4	1,262.7
NAV | Fair value measured on a recurring basis | Other long-term investments
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Other long-term investments	1,110.4	$ 864.0
Subsidiary - HG Global
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Purchase of surplus notes	$ 503.0
Subsidiary - HG Global | Maximum
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Percentage of par value of policy reinsured	15.00%
Subsidiary - Ark Insurance Holdings Limited
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Basic ownership	72.10%	72.10%
Ownership percentage, after equity incentives	61.90%	61.90%
Subsidiary - WM Outrigger Re | Preferred stocks
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Basic ownership	100.00%	100.00%
Subsidiary - HG Global | Preferred stocks
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Basic ownership	96.90%	96.90%
Subsidiary - HG Global | Common Stock
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Basic ownership	88.40%	88.40%
Kudu (3)
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Basic ownership	91.20%
Ownership percentage basic units outstanding		90.40%
Ownership percentage, fully diluted	77.90%	77.00%
Subsidiary - Distinguished Managing General Agent
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Basic ownership	55.50%			55.50%
Ownership percentage, fully diluted	43.60%			43.60%