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Reinsurance - Narrative (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Concentration Risk [Line Items]
Plus reinsurance recoverable on unpaid losses at period end	$ 1,767.1	$ 1,515.2	$ 1,369.5	$ 560.7
Reinsurance recoverables, uncollateralized	$ 1,237.8	$ 1,001.9
AM Best, A Rating | Standard & Poor's, A Plus Rating | Lloyd's
Concentration Risk [Line Items]
Concentration Risk, Percentage	14.10%	13.80%
AM Best, A Plus Rating | Standard & Poor's, AA- Rating | Munich Re
Concentration Risk [Line Items]
Concentration Risk, Percentage	16.80%	17.00%
AM Best, A Plus Rating | Standard & Poor's, AA- Rating | Scor Global P&C SE
Concentration Risk [Line Items]
Concentration Risk, Percentage	8.90%
AM Best, A Plus Rating | Standard & Poor's, A Plus Rating | Aioi Nissay Dowa
Concentration Risk [Line Items]
Concentration Risk, Percentage		7.60%

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Reinsurance Recoverables, Uncollateralized

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Amount, after valuation allowance, recoverable under reinsurance contracts including premium paid under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves.

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