Variable Interest Entities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Variable Interest Entity [Line Items]
Total assets	$ 18,676,196,000		$ 15,226,131,000
Total liabilities	11,579,416,000		9,538,250,000
Ceded premiums written	1,178,525,000		926,215,000	$ 839,264,000
Ceded premiums earned	1,095,886,000		833,929,000	628,675,000
Investments in other ventures, under equity method (Note 4)	115,172,000		101,974,000
Redeemable noncontrolling interests	2,051,700,000		1,296,506,000
Upsilon RFO Re Ltd. | Primary Beneficiary
Variable Interest Entity [Line Items]
Return of capital			84,300,000
Total assets	2,200,000,000		1,200,000,000
Total liabilities	2,200,000,000		1,200,000,000
Upsilon RFO Re Ltd. | Primary Beneficiary | Non-voting preference shares
Variable Interest Entity [Line Items]
Non-voting preference shares issued to third parties			180,600,000
Investment in variable interest entity	$ 109,800,000		27,200,000
Upsilon RFO Re Ltd. | Primary Beneficiary | RenaissanceRe Holdings Ltd.
Variable Interest Entity [Line Items]
Return of capital			$ 33,000,000
Variable interest entity, ownership percentage	14.00%		20.80%
Upsilon RFO Re Ltd. | Primary Beneficiary | Investor | Non-voting preference shares
Variable Interest Entity [Line Items]
Investment in variable interest entity	$ 856,700,000	$ 400,000,000	$ 7,500,000
Mona Lisa Re Ltd. | Not Primary Beneficiary
Variable Interest Entity [Line Items]
Total assets	41,000		25,900,000
Total liabilities	41,000		25,900,000
Mona Lisa Re Ltd. | Not Primary Beneficiary | Renaissance Reinsurance Ltd.
Variable Interest Entity [Line Items]
Ceded premiums written	200,000		400,000
Ceded premiums earned	200,000		4,100,000
Mona Lisa Re Ltd. | Not Primary Beneficiary | DaVinci Reinsurance Ltd.
Variable Interest Entity [Line Items]
Ceded premiums written	200,000		400,000
Ceded premiums earned	200,000		2,900,000
Fibonacci Reinsurance Ltd. | Not Primary Beneficiary | Other investments
Variable Interest Entity [Line Items]
Fair value of investment	6,000,000		14,100,000
Renaissance Reinsurance Ltd. | Not Primary Beneficiary | Fibonacci Reinsurance Ltd.
Variable Interest Entity [Line Items]
Ceded premiums written	9,100,000		9,000,000
Ceded premiums earned	10,000,000		8,200,000
Langhorne Holdings LLC | Not Primary Beneficiary
Variable Interest Entity [Line Items]
Investments in other ventures, under equity method (Note 4)	1,300,000		600,000
Langhorne Partners | Not Primary Beneficiary
Variable Interest Entity [Line Items]
Investments in other ventures, under equity method (Note 4)	100,000		0
Vermeer Reinsurance Ltd.
Variable Interest Entity [Line Items]
Redeemable noncontrolling interests	599,989,000		$ 0	$ 0
Vermeer Reinsurance Ltd. | Primary Beneficiary
Variable Interest Entity [Line Items]
Total assets	600,400,000
Total liabilities	$ 400,000