Reinsurance Reinsurance (Ceded Reinsurance Receivable Assets) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 782,027	$ 683,972
Reinsurance Recoverables, Percentage of Total	100.00%	100.00%
Other Reinsurers [Member]
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 127,808	$ 118,679
Reinsurance Recoverables, Percentage of Total	16.20%	17.30%
AM Best, A+ Rating [Member] | Reinsurer A [Member]
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 301,478	$ 240,894
Reinsurance Recoverables, Percentage of Total	38.60%	35.20%
AM Best, A+ Rating [Member] | Reinsurer B [Member]
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 203,898	$ 183,881
Reinsurance Recoverables, Percentage of Total	26.10%	26.90%
AM Best, A+ Rating [Member] | Reinsurer C [Member]
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 67,723	$ 68,832
Reinsurance Recoverables, Percentage of Total	8.70%	10.10%
AM Best, A++ Rating [Member] | Reinsurer D [Member]
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 40,592	$ 36,202
Reinsurance Recoverables, Percentage of Total	5.20%	5.30%
AM Best, A Rating [Member] | Reinsurer E [Member]
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 40,528	$ 35,484
Reinsurance Recoverables, Percentage of Total	5.20%	5.20%