Variable Interest Entities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Variable Interest Entity [Line Items]
Total assets	$ 15,226,131	$ 12,352,082
Total liabilities	9,538,250	6,309,911
Ceded premiums written	926,215	839,264	$ 595,127
Ceded premiums earned	833,929	628,675	$ 466,719
Investments in other ventures, under equity method (Note 5)	101,974	124,227
Upsilon RFO Re Ltd. | Primary Beneficiary
Variable Interest Entity [Line Items]
Return of capital	84,300	242,500
Total assets	1,200,000	193,000
Total liabilities	1,200,000	193,000
Upsilon RFO Re Ltd. | Primary Beneficiary | Non-voting preference shares
Variable Interest Entity [Line Items]
Non-voting preference shares issued to third parties	180,600	166,600
Investment in variable interest entity	27,200	55,200
Upsilon RFO Re Ltd. | Primary Beneficiary | RenaissanceRe Holdings Ltd.
Variable Interest Entity [Line Items]
Return of capital	$ 33,000	$ 59,800
Variable interest entity, ownership percentage	20.80%	28.80%
Upsilon RFO Re Ltd. | Primary Beneficiary | Investor | Non-voting preference shares
Variable Interest Entity [Line Items]
Investment in variable interest entity	$ 7,500
Mona Lisa Re Ltd. | Not Primary Beneficiary
Variable Interest Entity [Line Items]
Total assets	25,900	$ 184,200
Total liabilities	25,900	184,200
Mona Lisa Re Ltd. | Not Primary Beneficiary | Renaissance Reinsurance Ltd.
Variable Interest Entity [Line Items]
Ceded premiums written	400	7,400
Ceded premiums earned	4,100	7,300
Mona Lisa Re Ltd. | Not Primary Beneficiary | DaVinci Reinsurance Ltd.
Variable Interest Entity [Line Items]
Ceded premiums written	400	5,100
Ceded premiums earned	2,900	$ 5,000
Renaissance Reinsurance Ltd. | Primary Beneficiary | Fibonacci Reinsurance Ltd.
Variable Interest Entity [Line Items]
Ceded premiums written	9,000
Ceded premiums earned	8,200
Fibonacci Reinsurance Ltd. | Primary Beneficiary | Other Investments
Variable Interest Entity [Line Items]
Fair value of investment	14,100
Langhorne Holdings LLC | Not Primary Beneficiary
Variable Interest Entity [Line Items]
Investments in other ventures, under equity method (Note 5)	$ 600