Reinsurance (Ceded Reinsurance Receivable Assets) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 798,365	$ 683,972
Reinsurance Recoverables, Percentage of Total	100.00%	100.00%
Other Reinsurers [Member]
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 154,095	$ 118,679
Reinsurance Recoverables, Percentage of Total	19.30%	17.30%
AM Best, A+ Rating [Member] | Reinsurer A [Member]
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 291,029	$ 240,894
Reinsurance Recoverables, Percentage of Total	36.40%	35.20%
AM Best, A+ Rating [Member] | Reinsurer B [Member]
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 195,222	$ 183,881
Reinsurance Recoverables, Percentage of Total	24.40%	26.90%
AM Best, A+ Rating [Member] | Reinsurer C [Member]
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 67,691	$ 68,832
Reinsurance Recoverables, Percentage of Total	8.50%	10.10%
AM Best, A++ Rating [Member] | Reinsurer D [Member]
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 47,520	$ 36,202
Reinsurance Recoverables, Percentage of Total	6.00%	5.30%
AM Best, A Rating [Member] | Reinsurer E [Member]
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 42,808	$ 35,484
Reinsurance Recoverables, Percentage of Total	5.40%	5.20%