Reinsurance (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Reinsurance recoverable	$ 2,791,297	$ 2,372,221	$ 1,586,630
Allowance for Reinsurance Recoverable
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation allowance recorded against reinsurance recoverable	$ 7,300	$ 9,000
Reinsurance Recoverable | Largest reinsurance recoverable, Customer One
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Concentration risk percentage	12.70%	15.50%
Reinsurance Recoverable | Largest reinsurance recoverable, Customer Two
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Concentration risk percentage	7.20%	6.70%
Reinsurance Recoverable | Largest reinsurance recoverable, Customer Three
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Concentration risk percentage	7.00%	6.50%
Allowance for Reinsurance Recoverable | Largest reinsurance recoverable, Customer One
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Concentration risk percentage	18.10%	16.20%
Allowance for Reinsurance Recoverable | Largest reinsurance recoverable, Customer Two
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Concentration risk percentage	7.90%	14.80%
Allowance for Reinsurance Recoverable | Largest reinsurance recoverable, Customer Three
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Concentration risk percentage	7.20%	12.30%
Ceded Credit Risk, Secured | Reinsurance Recoverable | Reinsurer Concentration Risk
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Concentration risk percentage	57.50%	60.80%
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	41.00%	38.00%
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	1.50%	1.20%