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Reinsurance (Narrative) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Ceded Credit Risk [Line Items]
Reinsurance recoverable	$ 1,454,991	$ 370,586	$ 1,586,630
Allowance for reinsurance recoverable
Ceded Credit Risk [Line Items]
Valuation allowances and reserves balance	$ 7,800		$ 7,000
Customer Concentration Risk, Customer One | Reinsurance recoverable
Ceded Credit Risk [Line Items]
Concentration risk, percentage	10.00%	10.40%
Customer Concentration Risk, Customer One | Allowance for reinsurance recoverable
Ceded Credit Risk [Line Items]
Concentration risk, percentage	16.00%	11.10%
Customer Concentration Risk, Customer Two | Reinsurance recoverable
Ceded Credit Risk [Line Items]
Concentration risk, percentage	9.40%	7.50%
Customer Concentration Risk, Customer Two | Allowance for reinsurance recoverable
Ceded Credit Risk [Line Items]
Concentration risk, percentage	15.60%	9.20%
Customer Concentration Risk, Customer Three | Reinsurance recoverable
Ceded Credit Risk [Line Items]
Concentration risk, percentage	7.40%	7.30%
Customer Concentration Risk, Customer Three | Allowance for reinsurance recoverable
Ceded Credit Risk [Line Items]
Concentration risk, percentage	14.40%	8.40%
Ceded Credit Risk, Secured | Reinsurer Concentration Risk | Reinsurance recoverable
Ceded Credit Risk [Line Items]
Concentration risk, percentage	48.20%	54.50%
Ceded Credit Risk, Unsecured | Reinsurer Concentration Risk | Reinsurance recoverable | Standard & Poor's, A- Rating
Ceded Credit Risk [Line Items]
Concentration risk, percentage	50.60%	44.50%
Ceded Credit Risk, Unsecured | Reinsurer Concentration Risk | Reinsurance recoverable | Standard & Poor's, BBB Rating
Ceded Credit Risk [Line Items]
Concentration risk, percentage	1.20%	1.00%

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6351-108592

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-Publisher FASB
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-Section 50
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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 20
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-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6824-158387

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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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