For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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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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Total of allowances and reserves, the valuation and qualifying accounts that are either netted against the cost of an asset (in order to value it at its carrying value) or that reflect a liability established to represent expected future costs.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Reinsurance (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
Reinsurance (Narrative) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation and Qualifying Accounts Disclosure [Line Items]
Reinsurance recoverable	$ 1,586,630	$ 279,564	$ 134,526
Allowance for Reinsurance Recoverable
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation allowance recorded against reinsurance recoverable	$ 7,000	$ 4,200
Reinsurance Recoverable \| Largest reinsurance recoverable, Customer One
Valuation and Qualifying Accounts Disclosure [Line Items]
Concentration risk percentage	10.40%	27.10%
Reinsurance Recoverable \| Largest reinsurance recoverable, Customer Two
Valuation and Qualifying Accounts Disclosure [Line Items]
Concentration risk percentage	7.50%	19.90%
Reinsurance Recoverable \| Largest reinsurance recoverable, Customer Three
Valuation and Qualifying Accounts Disclosure [Line Items]
Concentration risk percentage	7.30%	7.70%
Allowance for Reinsurance Recoverable \| Largest reinsurance recoverable, Customer One
Valuation and Qualifying Accounts Disclosure [Line Items]
Concentration risk percentage	11.10%	27.10%
Allowance for Reinsurance Recoverable \| Largest reinsurance recoverable, Customer Two
Valuation and Qualifying Accounts Disclosure [Line Items]
Concentration risk percentage	9.20%	17.90%
Allowance for Reinsurance Recoverable \| Largest reinsurance recoverable, Customer Three
Valuation and Qualifying Accounts Disclosure [Line Items]
Concentration risk percentage	8.40%	5.60%
Ceded Credit Risk, Secured \| Reinsurance Recoverable \| Reinsurer Concentration Risk
Valuation and Qualifying Accounts Disclosure [Line Items]
Concentration risk percentage	54.50%	47.30%
Rated A- or higher \| Ceded Credit Risk, Unsecured \| Reinsurance Recoverable \| Reinsurer Concentration Risk
Valuation and Qualifying Accounts Disclosure [Line Items]
Concentration risk percentage	44.50%	52.60%
Rated lower than A- \| Ceded Credit Risk, Unsecured \| Reinsurance Recoverable \| Reinsurer Concentration Risk
Valuation and Qualifying Accounts Disclosure [Line Items]
Concentration risk percentage	1.00%	0.10%