Contingent Liabilities - Narrative (Details) $ in Millions	Sep. 30, 2017 USD ($)
JRG Reinsurance Company, Ltd. | Letter of credit 100 Million
Contingent Liabilities [Line Items]
Letters of credit facility, amount	$ 100.0
Amount of letters of credit issued	85.6
Assets deposited for securing letters of credit	95.8
JRG Reinsurance Company, Ltd. | Letter of credit 102.5 Million
Contingent Liabilities [Line Items]
Letters of credit facility, amount	102.5
Amount of letters of credit issued	47.6
Assets deposited for securing letters of credit	61.6
James River Insurance Co
Contingent Liabilities [Line Items]
Total amount deposited in trust accounts	670.1
Third-party Reinsureds | JRG Reinsurance Company, Ltd.
Contingent Liabilities [Line Items]
Total amount deposited in trust accounts	$ 269.8