Commitments and Contingent Liabilities - Narrative (Details)	Dec. 31, 2019 USD ($) letter_of_credit_facility	Dec. 31, 2018 USD ($)
Contingent Liabilities [Line Items]
Amount of collateral held	$ 1,199,164,000	$ 0
JRG Reinsurance Company, Ltd.
Contingent Liabilities [Line Items]
Letter of credit facilities | letter_of_credit_facility	3
Total amount deposited in trust accounts	$ 283,200,000
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	22,700,000
Assets deposited for securing letters of credit	35,900,000
JRG Reinsurance Company, Ltd. | 2013 Facility
Contingent Liabilities [Line Items]
Amount of letters of credit issued	77,600,000
Assets deposited for securing letters of credit	93,900,000
JRG Reinsurance Company, Ltd. | 2017 Facility
Contingent Liabilities [Line Items]
Amount of letters of credit issued	21,900,000
Assets deposited for securing letters of credit	51,500,000
Raiser
Contingent Liabilities [Line Items]
Amount of collateral held	$ 1,199,200,000