Reinsurance (Ceded Reinsurance Receivable Assets) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 814,806	$ 757,572
Reinsurance Recoverables, Percentage of Total	100.00%	100.00%
Other Reinsurers [Member]
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 127,715	$ 114,723
Reinsurance Recoverables, Percentage of Total	15.60%	15.20%
AM Best, A+ Rating [Member] | Reinsurer A [Member]
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 324,370	$ 303,036
Reinsurance Recoverables, Percentage of Total	39.80%	40.00%
AM Best, A+ Rating [Member] | Reinsurer B [Member]
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 199,748	$ 193,324
Reinsurance Recoverables, Percentage of Total	24.50%	25.50%
AM Best, A+ Rating [Member] | Reinsurer E [Member]
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 41,362	$ 40,004
Reinsurance Recoverables, Percentage of Total	5.10%	5.30%
AM Best, A++ Rating [Member] | Reinsurer D [Member]
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 50,244	$ 36,600
Reinsurance Recoverables, Percentage of Total	6.20%	4.80%
AM Best, A Rating [Member] | Reinsurer C [Member]
Ceded Credit Risk [Line Items]
Reinsurance ceded receivables	$ 71,367	$ 69,885
Reinsurance Recoverables, Percentage of Total	8.80%	9.20%