Reinsurance - Ceded Credit Risk (Details) - Reinsurance Recoverable [Member]	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
A or better rating
Ceded Credit Risk [Line Items]
Maximum percentage of ceded written premium	95.00%	96.00%
Ceded Credit Risk, Concentrated Credit Risk [Member]
Ceded Credit Risk [Line Items]
Maximum percentage of ceded written premium	100.00%
Ceded Credit Risk, Concentrated Credit Risk [Member] | AAA
Ceded Credit Risk [Line Items]
Maximum percentage of ceded written premium	2.80%
Ceded Credit Risk, Concentrated Credit Risk [Member] | AA
Ceded Credit Risk [Line Items]
Maximum percentage of ceded written premium	44.50%
Ceded Credit Risk, Concentrated Credit Risk [Member] | AA- | Munich Reinsurance Company
Ceded Credit Risk [Line Items]
Maximum percentage of ceded written premium	25.60%
Ceded Credit Risk, Concentrated Credit Risk [Member] | AA- | Swiss Reinsurance Company
Ceded Credit Risk [Line Items]
Maximum percentage of ceded written premium	7.20%
Ceded Credit Risk, Concentrated Credit Risk [Member] | A+ | Transatlantic Reinsurance Company [Member]
Ceded Credit Risk [Line Items]
Maximum percentage of ceded written premium	4.10%
Ceded Credit Risk, Concentrated Credit Risk [Member] | A+ | Arch Reinsurance Co [Member]
Ceded Credit Risk [Line Items]
Maximum percentage of ceded written premium	3.70%
Ceded Credit Risk, Concentrated Credit Risk [Member] | A+ | Lloyds Syndicate
Ceded Credit Risk [Line Items]
Maximum percentage of ceded written premium	15.90%
Ceded Credit Risk, Concentrated Credit Risk [Member] | A+ | Axis Reinsurance Co [Member]
Ceded Credit Risk [Line Items]
Maximum percentage of ceded written premium	3.00%
Ceded Credit Risk, Concentrated Credit Risk [Member] | A
Ceded Credit Risk [Line Items]
Maximum percentage of ceded written premium	48.10%
Ceded Credit Risk, Concentrated Credit Risk [Member] | BBB
Ceded Credit Risk [Line Items]
Maximum percentage of ceded written premium	0.60%
Ceded Credit Risk, Concentrated Credit Risk [Member] | Standard Poors BB And Below Rating [Member]
Ceded Credit Risk [Line Items]
Maximum percentage of ceded written premium	0.60%
Ceded Credit Risk, Concentrated Credit Risk [Member] | Captives
Ceded Credit Risk [Line Items]
Maximum percentage of ceded written premium	3.30%
Ceded Credit Risk, Concentrated Credit Risk [Member] | Not Rated NR [Member]
Ceded Credit Risk [Line Items]
Maximum percentage of ceded written premium	0.10%
Minimum | Ceded Credit Risk, Concentrated Credit Risk [Member]
Ceded Credit Risk [Line Items]
Maximum percentage of ceded written premium	3.00%