Segment Information - Statement of earnings by segment (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Segment Reporting Information [Line Items]
Gross premiums written										$ 13,890,924		$ 10,978,925		$ 8,094,926
Net premiums written										10,242,683		7,951,063		5,944,841
Net premiums earned										9,777,934		8,226,425		5,895,070
Net losses and loss expenses										6,101,079	[1]	4,882,197	[1]	3,501,356
Acquisition costs										1,620,671		1,317,448		738,118
Operating expenses	[2]									1,508,859		1,439,371		1,069,689
Underwriting profit (loss)										547,325		587,409		585,907
Net investment income	[3]													721,016
Net results from structured products										14,280	[4]	17,991	[5]	54,209	[6]
Net fee income and other	[7]									(13,315)		(13,356)		(6,891)
Loss (gain) sale of subsidiaries										(7,088)		0		666,423
Total net realized gains (losses) on investments, and net unrealized gains (losses) on investments														122,991
Net realized and unrealized (gains) losses on derivative instruments										2,521		53,123		29,886
Net income (loss) from investment fund affiliates and operating affiliates										115,155		118,060		203,034
Gain on sale of operating affiliate										0		340,407		0
Extinguishment of debt										0		5,592		0
Exchange (gains) losses										(9,578)		22,504		(37,568)
Corporate operating expenses										505,483		480,755		220,165
Contribution from P&C and Corporate and Other										782,332		1,457,199		486,956
Interest expense										170,058		163,021	[8]	121,221	[8]
Non-controlling interests										129,177		106,187		80,498
Provision (benefit) for income tax										42,129		(19,161)		96,897
Net income (loss) attributable to common shareholders										440,968		1,207,152		188,340
Exclude Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	[3]									618,384		624,440
Total net realized gains (losses) on investments, and net unrealized gains (losses) on investments										112,689		19,997
Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income										154,751		187,489		129,575
Total net realized gains (losses) on investments, and net unrealized gains (losses) on investments										259,449		182,181		(15,529)
Net realized and unrealized (gains) losses on derivative instruments										(540,090)		(151,691)		(488,222)
Corporate and other
Segment Reporting Information [Line Items]
Gross premiums written	[9]									265,315		309,916		333,436
Net premiums written	[9]									12,047		62,239		177,632
Net premiums earned	[9]									12,047		62,241		177,632
Net losses and loss expenses	[9]									28,244	[1]	115,997	[1]	242,963
Acquisition costs	[9]									5,775	[1]	10,794	[1]	14,115
Operating expenses	[2],[9]									1,356		1,232		10,693
Underwriting profit (loss)	[9]									(23,328)		(65,782)		(90,139)
Net investment income	[3],[9]													146,558
Net results from structured products	[9]									0	[4]	0	[5]	0	[6]
Net fee income and other	[7],[9]									1,911		622		360
Loss (gain) sale of subsidiaries										(7,088)	[9]			666,423
Total net realized gains (losses) on investments, and net unrealized gains (losses) on investments	[9]													3,625
Net realized and unrealized (gains) losses on derivative instruments	[9]									2,521		53,123		29,886
Net income (loss) from investment fund affiliates and operating affiliates	[9]									115,155		118,060		203,034
Gain on sale of operating affiliate												340,407
Extinguishment of debt												5,592
Exchange (gains) losses	[9]									(9,578)		22,504		(37,568)
Corporate operating expenses	[9]									505,483		480,755		220,165
Contribution from P&C and Corporate and Other	[9]									(501,563)		201,538		(929,872)
Interest expense	[8],[9]									170,058		163,021		121,221
Non-controlling interests	[9]									129,177		106,187		80,498
Provision (benefit) for income tax	[9]									42,129		(19,161)		96,897
Corporate and other | Exclude Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	[3],[9]									31,726		40,569
Total net realized gains (losses) on investments, and net unrealized gains (losses) on investments	[9]									(14,844)		5,411
Corporate and other | Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income										154,751	[9]	187,489		129,575
Total net realized gains (losses) on investments, and net unrealized gains (losses) on investments										259,452	[9]	182,181	[9]	(15,529)
Net realized and unrealized (gains) losses on derivative instruments										(540,090)	[9]	(151,691)	[9]	(488,222)
Operating Segments [Member] | Property and casualty operations
Segment Reporting Information [Line Items]
Gross premiums written										13,625,609		10,669,009		7,761,490
Net premiums written										10,230,636		7,888,824		5,767,209
Net premiums earned		$ 2,454,060	$ 2,431,677	$ 2,528,704	$ 2,351,446	$ 2,375,155	$ 2,405,740	$ 2,063,795	$ 1,319,494	9,765,887		8,164,184		5,717,438
Net losses and loss expenses										6,072,835	[1]	4,766,200	[1]	3,258,393
Acquisition costs										1,614,896	[1]	1,306,654	[1]	724,003
Operating expenses	[2]									1,507,503		1,438,139		1,058,996
Underwriting profit (loss)		126,420	166,851	101,893	175,489	183,458	114,118	208,779	146,836	570,653		653,191		676,046
Net investment income	[3]													574,458
Net results from structured products										14,280	[4]	17,991	[5]	54,209	[6]
Net fee income and other	[7]									(15,226)		(13,978)		(7,251)
Total net realized gains (losses) on investments, and net unrealized gains (losses) on investments														119,366
Net realized and unrealized (gains) losses on derivative instruments										0				0
Contribution from P&C and Corporate and Other										$ 1,283,895		$ 1,255,661		$ 1,416,828
Ratios - P&C operations:
Loss and loss expense ratio	[10]									62.20%		58.40%		57.00%
Underwriting expense ratio	[10]									32.00%		33.60%		31.20%
Combined ratio	[10]									94.20%		92.00%		88.20%
Operating Segments [Member] | Property and casualty operations | Exclude Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	[3]									$ 586,658		$ 583,871
Total net realized gains (losses) on investments, and net unrealized gains (losses) on investments										127,533		14,586
Operating Segments [Member] | Property and casualty operations | Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Total net realized gains (losses) on investments, and net unrealized gains (losses) on investments										(3)
Operating Segments [Member] | Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written										9,650,503		8,395,846		$ 5,976,011
Net premiums written										6,715,969		5,859,934		4,134,151
Net premiums earned										6,651,495		5,648,482		4,026,713
Net losses and loss expenses										4,320,737	[1]	3,614,048	[1]	2,543,108
Acquisition costs										897,308	[1]	704,364	[1]	393,319
Operating expenses	[2]									1,224,233		1,154,760		865,592
Underwriting profit (loss)										209,217		175,310		224,694
Net results from structured products										8,495	[4]	12,185	[5]	43,710	[6]
Net fee income and other	[7]									$ (18,506)		$ (16,936)		$ (10,051)
Ratios - P&C operations:
Loss and loss expense ratio	[10]									65.00%		64.00%		63.20%
Underwriting expense ratio	[10]									31.90%		32.90%		31.20%
Combined ratio	[10]									96.90%		96.90%		94.40%
Operating Segments [Member] | Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written										$ 3,975,106		$ 2,273,163		$ 1,785,479
Net premiums written										3,514,667		2,028,890		1,633,058
Net premiums earned										3,114,392		2,515,702		1,690,725
Net losses and loss expenses										1,752,098	[1]	1,152,152	[1]	715,285
Acquisition costs										717,588	[1]	602,290	[1]	330,684
Operating expenses	[2]									283,270		283,379		193,404
Underwriting profit (loss)										361,436		477,881		451,352
Net results from structured products										5,785	[4]	5,806	[5]	10,499	[6]
Net fee income and other	[7]									$ 3,280		$ 2,958		$ 2,800
Ratios - P&C operations:
Loss and loss expense ratio	[10]									56.30%		45.80%		42.30%
Underwriting expense ratio	[10]									32.10%		35.20%		31.00%
Combined ratio	[10]									88.40%		81.00%		73.30%
Operating Segments [Member] | Corporate and other
Segment Reporting Information [Line Items]
Net premiums earned		$ 3,015	$ 2,360	$ 3,508	$ 3,164	$ 11,665	$ 17,812	$ 18,258	$ 14,506

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

[4] The net results from P&C structured products include net investment income and interest expense of $54.0 million and $39.7 million, respectively.

[5] The net results from P&C structured products include net investment income and interest expense of $60.4 million and $42.2 million, respectively.

[6] The net results from P&C structured products include net investment income and interest expense of $68.0 million and $12.9 million, respectively.

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

[8] Interest expense excludes interest expense related to deposit liabilities recorded in the Insurance and Reinsurance segments.

[9] Corporate and Other includes other items of our revenue and expenditures that are not evaluated at the segment level for reporting purposes, as well as the Company's Run-Off Life Operations.

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