Reinsurance (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Reinsurance recoverable	$ 2,372,221	$ 1,586,630	$ 279,564
Allowance for Reinsurance Recoverable
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation allowance recorded against reinsurance recoverable	$ 9,000	$ 7,000
Reinsurance Recoverable | Largest reinsurance recoverable, Customer One
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Concentration risk percentage	15.50%	10.40%
Reinsurance Recoverable | Largest reinsurance recoverable, Customer Two
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Concentration risk percentage	6.70%	7.50%
Reinsurance Recoverable | Largest reinsurance recoverable, Customer Three
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Concentration risk percentage	6.50%	7.30%
Allowance for Reinsurance Recoverable | Largest reinsurance recoverable, Customer One
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Concentration risk percentage	16.20%	11.10%
Allowance for Reinsurance Recoverable | Largest reinsurance recoverable, Customer Two
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Concentration risk percentage	14.80%	9.20%
Allowance for Reinsurance Recoverable | Largest reinsurance recoverable, Customer Three
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Concentration risk percentage	12.30%	8.40%
Ceded Credit Risk, Secured | Reinsurance Recoverable | Reinsurer Concentration Risk
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Concentration risk percentage	60.80%	54.50%
Rated A- or higher | Ceded Credit Risk, Unsecured | Reinsurance Recoverable | Reinsurer Concentration Risk
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Concentration risk percentage	38.00%	44.50%
Rated lower than A- | Ceded Credit Risk, Unsecured | Reinsurance Recoverable | Reinsurer Concentration Risk
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Concentration risk percentage	1.20%	1.00%