Contingent Liabilities - Narrative (Details)	Sep. 30, 2020 USD ($) credit_facility	Dec. 31, 2019 USD ($)
Contingent Liabilities [Line Items]
Amount of collateral held	$ 940,221,000	$ 1,199,164,000
JRG Reinsurance Company, Ltd.
Contingent Liabilities [Line Items]
Number of credit facilities | credit_facility	3
JRG Reinsurance Company, Ltd. | Letter of credit 75 Million
Contingent Liabilities [Line Items]
Letters of credit facility, amount	$ 75,000,000.0
Amount of letters of credit issued	12,200,000
Assets deposited for securing letters of credit	25,200,000
JRG Reinsurance Company, Ltd. | Letter of credit 102.5 Million
Contingent Liabilities [Line Items]
Letters of credit facility, amount	102,500,000
Amount of letters of credit issued	76,000,000.0
Assets deposited for securing letters of credit	96,700,000
JRG Reinsurance Company, Ltd. | Letter of credit 100 Million
Contingent Liabilities [Line Items]
Amount of letters of credit issued	17,500,000
Assets deposited for securing letters of credit	28,300,000
Line of credit (up to)	100,000,000.0
Third-party Reinsureds | JRG Reinsurance Company, Ltd.
Contingent Liabilities [Line Items]
Collateral trust arrangement amount	302,100,000
Raiser
Contingent Liabilities [Line Items]
Amount of collateral held	$ 940,200,000