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Reinsurance - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Concentration Risk [Line Items]
Plus reinsurance recoverable on unpaid losses at period end	$ 2,122.9	$ 2,077.6	$ 1,611.3	$ 1,515.2
Reinsurance recoverables, uncollateralized	$ 1,642.0	$ 1,497.8
AM Best, A Rating | Standard & Poor's, A Plus Rating | Lloyd's
Concentration Risk [Line Items]
Concentration Risk, Percentage	9.50%	10.20%
AM Best, A Plus Rating | Standard & Poor's, AA- Rating | Munich Re
Concentration Risk [Line Items]
Concentration Risk, Percentage	16.10%	15.70%
AM Best, A Plus Rating | Standard & Poor's, A Plus Rating | Everest
Concentration Risk [Line Items]
Concentration Risk, Percentage	10.00%	10.20%

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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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