Reinsurance (Ceded Reinsurance Receivable Assets) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 769,324	$ 782,027
Reinsurance Recoverables, Percentage of Total	100.00%	100.00%
Other Reinsurers [Member]
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 117,942	$ 127,808
Reinsurance Recoverables, Percentage of Total	15.30%	16.20%
AM Best, A+ Rating [Member] | Reinsurer A [Member]
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 303,942	$ 301,478
Reinsurance Recoverables, Percentage of Total	39.50%	38.60%
AM Best, A+ Rating [Member] | Reinsurer B [Member]
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 203,702	$ 203,898
Reinsurance Recoverables, Percentage of Total	26.50%	26.10%
AM Best, A+ Rating [Member] | Reinsurer D [Member]
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 41,049	$ 40,528
Reinsurance Recoverables, Percentage of Total	5.30%	5.20%
AM Best, A Rating [Member] | Reinsurer C [Member]
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 67,599	$ 67,723
Reinsurance Recoverables, Percentage of Total	8.80%	8.70%
AM Best, A++ Rating [Member] | Reinsurer E [Member]
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 35,090	$ 40,592
Reinsurance Recoverables, Percentage of Total	4.60%	5.20%