Contingent Liabilities - Narrative (Details) $ in Millions	6 Months Ended
	Jun. 30, 2017 USD ($) letter_of_credit
JRG Reinsurance Company, Ltd.
Contingent Liabilities [Line Items]
Number of letter of credit facilities | letter_of_credit	2
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	85.1
Assets deposited for securing letters of credit	104.3
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	45.2
Assets deposited for securing letters of credit	64.0
James River Insurance Co
Contingent Liabilities [Line Items]
Total amount deposited in trust accounts	549.8
Third-party Reinsureds | JRG Reinsurance Company, Ltd.
Contingent Liabilities [Line Items]
Total amount deposited in trust accounts	$ 251.4