Contingent Liabilities - Narrative (Details) $ in Millions	Mar. 31, 2018 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	52.2
Assets deposited for securing letters of credit	64.2
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	98.6
Assets deposited for securing letters of credit	125.9
JRG Reinsurance Company, Ltd. | Letter of credit 100 Million one
Contingent Liabilities [Line Items]
Letters of credit facility, amount	100.0
Amount of letters of credit issued	6.9
Assets deposited for securing letters of credit	10.0
James River Insurance Co
Contingent Liabilities [Line Items]
Total amount deposited in trust accounts	860.6
Third-party Reinsureds | JRG Reinsurance Company, Ltd.
Contingent Liabilities [Line Items]
Total amount deposited in trust accounts	$ 283.1