Variable Interest Entities (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Variable Interest Entity [Line Items]
Assets	$ 15,922,202,000		$ 15,226,131,000
Liabilities	10,060,585,000		9,538,250,000
Ceded premiums written	496,608,000	$ 377,954,000
Ceded premiums earned	235,233,000	191,122,000
Investments in other ventures, under equity method	120,232,000		101,974,000
Upsilon RFO Re Ltd. | Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Return of capital			84,300,000
Assets	1,400,000,000		1,200,000,000
Liabilities	1,400,000,000		1,200,000,000
Upsilon RFO Re Ltd. | Variable Interest Entity, Primary Beneficiary | Non-Voting Preference Shares
Variable Interest Entity [Line Items]
Issuance of equity to third party investors			180,600,000
Contributions to subsidiaries	$ 75,000,000		27,200,000
Upsilon RFO Re Ltd. | Variable Interest Entity, Primary Beneficiary | RenaissanceRe Holdings Ltd.
Variable Interest Entity [Line Items]
Return of capital			$ 33,000,000
Variable interest entity, ownership percentage	14.10%		20.80%
Upsilon RFO Re Ltd. | Variable Interest Entity, Primary Beneficiary | Third party investor | Non-Voting Preference Shares
Variable Interest Entity [Line Items]
Contributions to subsidiaries	$ 600,500,000		$ 7,500,000
Renaissance Reinsurance Ltd. | Variable Interest Entity, Primary Beneficiary | Fibonacci Reinsurance Ltd.
Variable Interest Entity [Line Items]
Ceded premiums written	4,100,000	7,400,000
Ceded premiums earned	1,200,000	1,800,000
Mona Lisa Re Ltd | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Assets	25,600,000		25,900,000
Liabilities	25,600,000		25,900,000
Mona Lisa Re Ltd | Variable Interest Entity, Not Primary Beneficiary | Renaissance Reinsurance Ltd.
Variable Interest Entity [Line Items]
Ceded premiums written	200,000	100,000
Ceded premiums earned	100,000	1,800,000
Mona Lisa Re Ltd | Variable Interest Entity, Not Primary Beneficiary | DaVinci Reinsurance Ltd.
Variable Interest Entity [Line Items]
Ceded premiums written	200,000	37,000
Ceded premiums earned	100,000	$ 1,300,000
Fibonacci Reinsurance Ltd. | Variable Interest Entity, Primary Beneficiary | Other Investments
Variable Interest Entity [Line Items]
Investment in participating notes	11,100,000		14,100,000
Langhorne Holdings LLC | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Investments in other ventures, under equity method	1,300,000		600,000
Langhorne Partners
Variable Interest Entity [Line Items]
Investments in other ventures, under equity method	$ 100,000
Langhorne Partners | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Investments in other ventures, under equity method			$ 0