Subsequent Event - Narrative (Details) $ in Millions	Jan. 01, 2021 USD ($)	Oct. 01, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Other Operations
Subsequent Event [Line Items]
Cash pre-funded/placed in escrow for Ark Transaction			$ 646.3	$ 0.0
Ark Insurance Holdings Limited
Subsequent Event [Line Items]
Cash pre-funded/placed in escrow for Ark Transaction			366.3
Ark Insurance Holdings Limited | Lloyd
Subsequent Event [Line Items]
Cash pre-funded/placed in escrow for Ark Transaction			$ 280.0
Ark Insurance Holdings Limited
Subsequent Event [Line Items]
Merger agreement, equity capital contributed		$ 605.4
Equity capital, pre money valuation amount		300.0
Additional equity purchase from shareholders		40.9
Investment, potential additional contribution of equity capital		$ 200.0
Ark Insurance Holdings Limited | Subsequent Event
Subsequent Event [Line Items]
Investment, potential additional contribution of equity capital	$ 200.0
Ownership interest after all transactions (as a percent)	72.00%
Ownership interest on fully-diluted basis (as a percent)	63.00%
Equity incentives, rate of return threshold	8.00%
Percent of shares outstanding	12.50%
Ark Insurance Holdings Limited | Subsequent Event | Tranche One
Subsequent Event [Line Items]
Invested capital return threshold, multiples	2.0
Ark Insurance Holdings Limited | Subsequent Event | Tranche Two
Subsequent Event [Line Items]
Invested capital return threshold, multiples	2.5
Ark Insurance Holdings Limited | Subsequent Event | Tranche Three
Subsequent Event [Line Items]
Invested capital return threshold, multiples	3.0
Ark Insurance Holdings Limited | Subsequent Event | Forecast
Subsequent Event [Line Items]
Ownership interest on fully-diluted basis (as a percent)	67.50%
Ownership interest after additional funding	77.10%