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Commitments and Contingencies - Additional Information (Details) $ / shares in Units, £ in Millions, SFr in Millions, $ in Millions, $ in Millions, $ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) Investment	Dec. 31, 2017 USD ($) Investment	Mar. 31, 2019 $ / shares	Dec. 31, 2018 GBP (£) Investment	Dec. 31, 2018 AUD ($) Investment	Dec. 31, 2018 CHF (SFr) Investment	Dec. 31, 2018 CAD ($) Investment	Dec. 31, 2018 SGD ($) Investment	Sep. 19, 2018 USD ($)	May 01, 2018 USD ($)	Dec. 31, 2017 GBP (£) Investment	Dec. 31, 2017 AUD ($) Investment	Dec. 31, 2017 CHF (SFr) Investment	Dec. 31, 2017 CAD ($) Investment	Dec. 31, 2017 SGD ($) Investment
Restricted Cash And Collateral [Line Items]
Restricted assets	$ 4,419.2	$ 4,638.8
Other investments (equity method)	$ 102.5	$ 0.0
Total as percent of investable assets	56.40%	53.40%
Investable assets held by the Company	$ 7,800.0	$ 8,700.0
Investable assets held by insurance regulators	$ 503.2	458.7
Minimum capital required | £				£ 0.4							£ 0.4
Percentage of reinsurance liabilities	100.00%			100.00%	100.00%	100.00%	100.00%	100.00%
Reinsurance liabilities	$ 1,311.4	1,268.4
Assets held-in-trust	1,336.4	1,350.9
Regulatory deposits	6.0	6.0
Deposit with states	6.1	6.1
Rental and premises expenses	27.3	26.3
Depreciation for fixed assets	29.7	34.6
Accumulated depreciation	$ 175.8	$ 146.2
Number of investments | Investment	2	2		2	2	2	2	2			2	2	2	2	2
Us Multi Beneficiary Trust Fund
Restricted Cash And Collateral [Line Items]
Minimum trust fund	$ 20.0
Assets held-in-trust	1,112.4	$ 1,333.6
Us Surplus Lines Trust Fund
Restricted Cash And Collateral [Line Items]
Assets held-in-trust	$ 198.8	195.0
Canadian Trust Fund
Restricted Cash And Collateral [Line Items]
Assets held-in-trust							$ 152.7							$ 169.9
Australian Trust Fund
Restricted Cash And Collateral [Line Items]
Assets held-in-trust					$ 209.3							$ 198.7
Swiss Trust Fund
Restricted Cash And Collateral [Line Items]
Assets held-in-trust | SFr						SFr 9.0							SFr 9.8
Singapore Trust Fund
Restricted Cash And Collateral [Line Items]
Assets held-in-trust								$ 135.9							$ 120.6
Bermuda
Restricted Cash And Collateral [Line Items]
Percentage of reinsurance liabilities	100.00%			100.00%	100.00%	100.00%	100.00%	100.00%
Reinsurance liabilities	$ 647.8	895.5
Limited Partner | Real estate fund
Restricted Cash And Collateral [Line Items]
Restricted assets	0.0	100.0
Other investments (equity method)	$ 102.5	$ 0.0							$ 13.8	$ 86.2
Subsequent Event
Restricted Cash And Collateral [Line Items]
Business acquisition, share price | $ / shares			$ 42.75

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