Debt and Credit Facilities (Credit Facilities Narrative) (Details)	Nov. 23, 2015 USD ($)	Dec. 31, 2019 USD ($)	Nov. 07, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 21, 2019 USD ($)	Mar. 22, 2019 USD ($)	Nov. 09, 2018 USD ($)	May 31, 2016 USD ($)	Sep. 17, 2010 USD ($)
Line of Credit Facility [Line Items]
Credit facilities outstanding				$ 1,110,756,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Letter of credit aggregate commitment							$ 500,000,000.0
Line of credit facility, maximum borrowing capacity							750,000,000.0
Debt to capital ratio		0.35
Minimum net worth requirements		$ 2,900,000,000
Unsecured Bilateral Facility | Wells Fargo | Letter of Credit
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity					$ 25,000,000.0
Credit facilities outstanding		0
Unsecured Bilateral Facility | Citibank Europe PLC | Letter of Credit
Line of Credit Facility [Line Items]
Credit facilities outstanding		270,700,000
Unsecured Bilateral Facility | Credit Suisse (Switzerland) Ltd. | Letter of Credit
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity						$ 125,000,000.0
Debt to capital ratio						0.35
Minimum net worth requirements						$ 3,000,000,000.0
Credit facilities outstanding		111,100,000
Secured Bilateral Facility | Wells Fargo | Letter of Credit
Line of Credit Facility [Line Items]
Credit facilities outstanding		31,500,000
Secured Bilateral Facility | Citibank Europe PLC | Letter of Credit
Line of Credit Facility [Line Items]
Credit facilities outstanding		266,500,000
Renaissance Reinsurance FAL Facility | Letter of Credit
Line of Credit Facility [Line Items]
Credit facilities outstanding		290,000,000.0
Bridge Loan | Revolving Credit Facility
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity							$ 50,000,000.0
Line of Credit | Bank of Montreal, Citibank Europe and ING Bank | Letter of Credit
Line of Credit Facility [Line Items]
Credit facilities outstanding				290,000,000
Line of Credit | Unsecured Bilateral Facility | Letter of Credit
Line of Credit Facility [Line Items]
Credit facilities outstanding				381,770,000
Line of Credit | Secured Bilateral Facility | Letter of Credit
Line of Credit Facility [Line Items]
Credit facilities outstanding				$ 298,063,000
Bilateral Facility Participants | Letter of Credit | Secured Bilateral Facility | Citibank Europe PLC
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity									$ 300,000,000.0
Loan agreement, remaining borrowing capacity		$ 33,500,000
Renaissance Reinsurance U.S. Inc. | Letter of Credit | Secured Bilateral Facility | Citibank Europe PLC
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity									$ 25,000,000.0
Syndicate 1458 | Letter of Credit | Renaissance Reinsurance FAL Facility | Bank of Montreal, Citibank Europe and ING Bank
Line of Credit Facility [Line Items]
Collateral amount, percent of aggregate outstanding letters of credit in case of Full Collateralization Event		100.00%
Syndicate 1458 | Line of Credit | Renaissance Reinsurance FAL Facility | Bank of Montreal, Citibank Europe and ING Bank
Line of Credit Facility [Line Items]
Aggregate amount of issuances	$ 140,000,000.0
Termination period from date of notice	4 years
Syndicate 1458 | Line of Credit | Renaissance Reinsurance FAL Facility | Bank of Montreal, Citibank Europe and ING Bank | Line of Credit
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity			$ 290,000,000.0					$ 255,000,000.0
Greater than or equal to | Syndicate 1458 | Letter of Credit | Renaissance Reinsurance FAL Facility | Bank of Montreal, Citibank Europe and ING Bank
Line of Credit Facility [Line Items]
Collateral amount, percent of aggregate outstanding letters of credit	60.00%
Less than | Syndicate 1458 | Letter of Credit | Renaissance Reinsurance FAL Facility | Bank of Montreal, Citibank Europe and ING Bank
Line of Credit Facility [Line Items]
Collateral amount, percent of aggregate outstanding letters of credit	100.00%
Letter of Credit | Top Layer Reinsurance Ltd.
Line of Credit Facility [Line Items]
Collateralized letter of credit and reimbursement agreement		$ 37,500,000
Capital Support Agreement | Letter of Credit | Top Layer Reinsurance Ltd.
Line of Credit Facility [Line Items]
Mandatory capital contribution in the event of capital and surplus reduction below a specified level		$ 50,000,000.0