Segment Information - Statement of earnings by segment (Details) - USD ($) $ in Thousands			3 Months Ended				9 Months Ended
		Jul. 07, 2017	Sep. 30, 2017		Sep. 30, 2016		Sep. 30, 2017		Sep. 30, 2016
Segment Reporting Information [Line Items]
Gross premiums written			$ 3,075,561		$ 2,783,603		$ 11,366,927		$ 10,811,152
Net premiums written			2,329,367		2,135,658		7,961,702		7,926,536
Net premiums earned			2,620,902		2,434,037		7,660,610		7,320,859
Net losses and loss expenses	[1]		3,077,315		1,497,678		6,204,740		4,522,968
Acquisition costs			445,685		403,888		1,315,138		1,227,675
Operating expenses	[2]		333,636		370,279		1,049,749		1,138,296
Underwriting profit (loss)			(1,235,734)		162,192		(909,017)		431,920
Net results from structured products			17,908	[3]	7,708	[4]	23,745	[5]	11,737	[6]
Net fee income and other			(2,019)	[7]	(3,271)	[8]	(1,801)	[9]	(8,324)	[10]
(Gain) Loss on sale of subsidiary			0		(3,670)		0		(3,670)
Extinguishment of debt		$ 1,582	1,582		0		1,582		0
Net realized gains (losses) on investments			43,274		143,676		137,188		341,043
Net realized and unrealized gains (losses) on derivative instruments			(20,434)		5,490		(28,409)		2,774
Net income (loss) from investment fund affiliates and operating affiliates			62,528		24,566		187,920		67,234
Exchange (gains) losses			55,017		(1,695)		46,038		(54,614)
Corporate operating expenses			62,232		126,308		263,267		372,003
Contribution from P&C and Corporate and Other			(1,094,509)		189,451		(444,847)		431,413
Interest expense		$ 300	39,509	[11]	43,025	[11]	123,181	[11]	128,371	[11]
Non-controlling interests			(30,197)		58,076		39,232		124,263
Income tax			(60,132)		17,749		(18,034)		42,511
Net income (loss) attributable to common shareholders			(1,043,689)		70,601		(589,226)		136,268
Excluding Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	[12]		159,665		156,706		477,174		470,537
Net realized gains (losses) on investments			31,726		58,395		85,113		69,447
Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income			30,796		38,937		95,599		119,643
Net realized gains (losses) on investments			11,548		85,281		52,075		271,596
Net realized and unrealized gains (losses) on derivative instruments			(31,662)		(225,610)		(116,359)		(691,432)
Corporate and Other
Segment Reporting Information [Line Items]
Gross premiums written	[13]		59,830		64,683		175,160		202,139
Net premiums written	[13]		3,146		2,360		9,280		9,032
Net premiums earned	[13]		3,146		2,360		9,280		9,032
Net losses and loss expenses	[1],[13]		10,592		5,875		25,478		16,294
Acquisition costs	[13]		1,126		1,002	[1]	3,715	[1]	4,286	[1]
Operating expenses	[2],[13]		186		142		697		765
Underwriting profit (loss)	[13]		(8,758)		(4,659)		(20,610)		(12,313)
Net results from structured products	[13]		0	[3]	0	[4]	0	[5]	0	[6]
Net fee income and other	[13]		(1,304)	[7]	595	[8]	(1,607)	[9]	1,068	[10]
(Gain) Loss on sale of subsidiary					(3,670)				(3,670)
Extinguishment of debt			1,582				1,582
Net realized and unrealized gains (losses) on derivative instruments	[13]		(20,434)		5,490		(28,409)		2,774
Net income (loss) from investment fund affiliates and operating affiliates	[13]		62,528		24,566		187,920		67,234
Exchange (gains) losses	[13]		55,017		(1,695)		46,038		(54,614)
Corporate operating expenses	[13]		62,232		126,308		263,267		372,003
Contribution from P&C and Corporate and Other	[13]		(69,476)		(190,290)		(128,837)		(538,703)
Interest expense	[11],[13]		39,509		43,025		123,181		128,371
Non-controlling interests	[13]		(30,197)		58,076		39,232		124,263
Income tax	[13]		(60,132)		17,749		(18,034)		42,511
Corporate and Other | Excluding Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	[12],[13]		7,533		8,127		22,917		24,185
Net realized gains (losses) on investments	[13]		(892)		(2,006)		(9,476)		(7,742)
Corporate and Other | Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	[13]		30,796		38,937		95,599		119,643
Net realized gains (losses) on investments	[13]		11,548		85,213		52,075		271,599
Net realized and unrealized gains (losses) on derivative instruments	[13]		(31,662)		(225,610)		(116,359)		(691,432)
Operating segments | Total P&C
Segment Reporting Information [Line Items]
Gross premiums written			3,015,731		2,718,920		11,191,767		10,609,013
Net premiums written			2,326,221		2,133,298		7,952,422		7,917,504
Net premiums earned			2,617,756		2,431,677		7,651,330		7,311,827
Net losses and loss expenses	[1]		3,066,723		1,491,803		6,179,262		4,506,674
Acquisition costs			444,559		402,886	[1]	1,311,423	[1]	1,223,389	[1]
Operating expenses	[2]		333,450		370,137		1,049,052		1,137,531
Underwriting profit (loss)			(1,226,976)		166,851		(888,407)		444,233
Net results from structured products			17,908	[3]	7,708	[4]	23,745	[5]	11,737	[6]
Net fee income and other			(715)	[7]	(3,866)	[8]	(194)	[9]	(9,392)	[10]
Net realized and unrealized gains (losses) on derivative instruments			0		0		0
Contribution from P&C and Corporate and Other			$ (1,025,033)		$ 379,741		$ (316,010)		$ 970,116
Ratios - P&C operations
Loss and loss expense ratio	[14]		117.20%		61.30%		80.80%		61.60%
Underwriting expense ratio	[14]		29.70%		31.80%		30.80%		32.30%
Combined ratio	[14]		146.90%		93.10%		111.60%		93.90%
Operating segments | Total P&C | Excluding Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	[12]		$ 152,132		$ 148,579		$ 454,257		$ 446,352
Net realized gains (losses) on investments			32,618		60,401		94,589		77,189
Operating segments | Total P&C | Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net realized gains (losses) on investments			0		68		0		(3)
Operating segments | Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written			2,311,090		2,153,379		7,582,060		7,169,363
Net premiums written			1,660,304		1,615,132		4,910,096		4,899,258
Net premiums earned			1,677,189		1,653,461		4,964,808		4,944,055
Net losses and loss expenses	[1]		1,724,755		1,068,268		3,862,205		3,163,599
Acquisition costs			228,188		221,137	[1]	670,579	[1]	679,009	[1]
Operating expenses	[2]		269,727		302,373		844,137		922,273
Underwriting profit (loss)			(545,481)		61,683		(412,113)		179,174
Net results from structured products			1,862	[3]	2,353	[4]	6,112	[5]	6,340	[6]
Net fee income and other			$ (1,578)	[7]	$ (4,486)	[8]	$ (2,437)	[9]	$ (12,097)	[10]
Ratios - P&C operations
Loss and loss expense ratio	[14]		102.80%		64.60%		77.80%		64.00%
Underwriting expense ratio	[14]		29.70%		31.70%		30.50%		32.40%
Combined ratio	[14]		132.50%		96.30%		108.30%		96.40%
Operating segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written			$ 704,641		$ 565,541		$ 3,609,707		$ 3,439,650
Net premiums written			665,917		518,166		3,042,326		3,018,246
Net premiums earned			940,567		778,216		2,686,522		2,367,772
Net losses and loss expenses	[1]		1,341,968		423,535		2,317,057		1,343,075
Acquisition costs			216,371		181,749	[1]	640,844	[1]	544,380	[1]
Operating expenses	[2]		63,723		67,764		204,915		215,258
Underwriting profit (loss)			(681,495)		105,168		(476,294)		265,059
Net results from structured products			16,046	[3]	5,355	[4]	17,633	[5]	5,397	[6]
Net fee income and other			$ 863	[7]	$ 620	[8]	$ 2,243	[9]	$ 2,705	[10]
Ratios - P&C operations
Loss and loss expense ratio	[14]		142.70%		54.40%		86.20%		56.70%
Underwriting expense ratio	[14]		29.80%		32.10%		31.50%		32.10%
Combined ratio	[14]		172.50%		86.50%		117.70%		88.80%

[1] The Company has reflected the amortization of certain fair value adjustments recorded in conjunction with the Catlin Acquisition within the respective segments.

[2] Operating expenses of the segments exclude Corporate operating expenses, shown separately.

[3] The net results from P&C structured products include net investment income and interest expense of $12.3 million and $(5.6) million, respectively.

[4] The net results from P&C structured products include net investment income and interest expense of $14.1 million and $6.4 million, respectively.

[5] The net results from P&C structured products include net investment income and interest expense of $39.2 million and $15.5 million, respectively.

[6] The net results from P&C structured products include net investment income and interest expense of $40.9 million and $29.1 million, respectively.

[7] Net fee income and other includes operating expenses of $13.5 million from the Company's loss prevention consulting services business.

[8] Net fee income and other includes operating expenses of $11.9 million from the Company's loss prevention consulting services business.

[9] Net fee income and other includes operating expenses of $37.2 million from the Company's loss prevention consulting services business.

[10] Net fee income and other includes operating expenses of $36.0 million from the Company's loss prevention consulting services business.

[11] Interest expense excludes interest expense related to structured products recorded in the Insurance and Reinsurance segments.

[12] Net investment income - excluding Life Funds Withheld Assets does not include net investment income related to the net results from structured products.

[13] Corporate and Other includes other items of our revenue and expenditures that are not evaluated at the segment level for reporting purposes. The underwriting profit (loss) and its components within this section are predominantly driven from the Company's run-off life operations.

[14] Ratios are based on net premiums earned from P&C operations.