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Commitments and Contingencies - Additional Information (Details) £ in Millions, SGD in Millions, SFr in Millions, CAD in Millions, AUD in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) Investment	Dec. 31, 2016 USD ($) Investment	Dec. 31, 2017 SGD Investment	Dec. 31, 2017 AUD Investment	Dec. 31, 2017 CAD Investment	Dec. 31, 2017 CHF (SFr) Investment	Dec. 31, 2017 GBP (£) Investment	Dec. 31, 2016 SGD Investment	Dec. 31, 2016 AUD Investment	Dec. 31, 2016 CAD Investment	Dec. 31, 2016 CHF (SFr) Investment	Dec. 31, 2016 GBP (£) Investment
Restricted Cash And Collateral [Line Items]
Total as percent of investable assets	53.40%	49.30%
Investable assets held by the Company	$ 8,700.0	$ 9,200.0
Restricted assets	4,638.8	4,535.7
Investable assets held by insurance regulators	$ 458.7	447.3
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	$ 1,268.4	1,101.5
Assets held-in-trust	1,350.9	1,351.2
Regulatory deposits	6.0	6.0
Deposit with states	6.1	6.0
Rental and premises expenses	26.3	24.1
Depreciation for fixed assets	34.6	22.6
Accumulated depreciation	$ 146.2	$ 111.6
Number of investments | Investment	2	3	2	2	2	2	2	3	3	3	3	3
Us Multi Beneficiary Trust Fund
Restricted Cash And Collateral [Line Items]
Minimum trust fund	$ 20.0
Assets held-in-trust	1,333.6	$ 1,267.4
Us Surplus Lines Trust Fund
Restricted Cash And Collateral [Line Items]
Assets held-in-trust	$ 195.0	185.7
Canadian Trust Fund
Restricted Cash And Collateral [Line Items]
Assets held-in-trust | CAD					CAD 169.9					CAD 249.2
Australian Trust Fund
Restricted Cash And Collateral [Line Items]
Assets held-in-trust | AUD				AUD 198.7					AUD 137.8
Swiss Trust Fund
Restricted Cash And Collateral [Line Items]
Assets held-in-trust | SFr						SFr 9.8					SFr 18.3
Singapore Trust Fund
Restricted Cash And Collateral [Line Items]
Assets held-in-trust | SGD			SGD 120.6					SGD 106.4
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	$ 895.5	953.2
Real estate fund
Restricted Cash And Collateral [Line Items]
Restricted assets	$ 100.0	$ 0.0

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