Held for Sale and Discontinued Operations - Narrative (Details) £ in Millions, $ in Millions						3 Months Ended	12 Months Ended
	Sep. 17, 2021 USD ($)	Apr. 12, 2021 USD ($)	Apr. 12, 2021 GBP (£)	Feb. 12, 2021 USD ($)	Aug. 20, 2020 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Discontinued Operations
Gain (loss) on sale of discontinued operations							$ 18.7	$ (2.3)	$ 0.8
Total assets							7,000.7	4,831.4
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Corporate Aircraft
Discontinued Operations
Proceeds from sale of property held-for-sale				$ 2.1
Gain on sale of aircraft				$ 0.5
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Connecticut
Discontinued Operations
Proceeds from sale of property held-for-sale					$ 2.3
Realized gain losses on sales of other real estate								(0.1)
Proceeds from sale of land	$ 0.7
Loss on sale of land	$ 0.1
Disposal Group, Held-for-sale, Not Discontinued Operations | Corporate Aircraft
Discontinued Operations
Assets held for sale								1.7
Disposal Group, Held-for-sale, Not Discontinued Operations | Connecticut
Discontinued Operations
Assets held for sale								0.7
Sirius Group
Discontinued Operations
Gain (loss) on sale of discontinued operations										$ 17.3
Sirius Group | Discontinued Operations, Disposed of by Sale
Discontinued Operations
Gain (loss) on sale of discontinued operations							17.6
Discontinued operations, liability								18.7
Increase in discontinued operations liability								$ 2.2
Sirius Group | Discontinued Operations, Disposed of by Sale | Foreign Currency Gain (Loss)
Discontinued Operations
Gain (loss) on sale of discontinued operations							1.1
Fresh Insurance Services Group Limited | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Discontinued Operations
Proceeds from sale of business		$ 1.5	£ 1.1
Fresh Insurance Services Group Limited | Disposal Group, Held-for-sale, Not Discontinued Operations
Discontinued Operations
Loss on sale						$ 28.7	(28.7)
Other Operations | Disposal Group, Held-for-sale, Not Discontinued Operations
Discontinued Operations
Total assets							$ 16.1