Segment Information - Statement of earnings by segment (Details) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended
		Jun. 30, 2017		Jun. 30, 2016		Jun. 30, 2017		Jun. 30, 2016
Segment Reporting Information [Line Items]
Gross premiums written		$ 3,612,949		$ 3,599,323		$ 8,291,366		$ 8,027,549
Net premiums written		2,649,265		2,726,119		5,632,335		5,790,878
Net premiums earned		2,516,917		2,532,212		5,039,708		4,886,822
Net losses and loss expenses	[1]	1,536,678		1,637,868		3,127,425		3,025,290
Acquisition costs		433,584		420,520		869,453		823,787
Operating expenses	[2]	358,394		376,133		716,113		768,017
Underwriting profit (loss)		188,261		97,691		326,717		269,728
Net results from structured products		2,923	[3]	2,059	[4]	5,837	[5]	4,029	[6]
Net fee income and other		(1,997)	[7]	(2,330)	[8]	218	[9]	(5,053)	[10]
Net realized gains (losses) on investments		56,628		104,617		93,914		197,367
Net realized and unrealized gains (losses) on derivative instruments		(906)		906		(7,975)		(2,716)
Net income (loss) from investment fund affiliates and operating affiliates		73,522		34,597		125,392		42,668
Gain on sale of operating affiliate								0
Exchange (gains) losses		(5,643)		(19,100)		(8,979)		(52,919)
Corporate operating expenses		102,162		133,185		201,035		245,695
Contribution from P&C and Corporate and Other		382,471		95,026		649,662		241,962
Interest expense	[11]	43,422		43,733		83,672		85,346
Non-controlling interests		8,423		5,044		69,429		66,187
Income tax		29,006		2,467		42,098		24,762
Net income (loss) attributable to common shareholders		301,620		43,782		454,463		65,667
Excluding Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	[12]	163,716		162,167		317,509		313,831
Net realized gains (losses) on investments		49,169		19,468		53,387		11,052
Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income		31,439		39,146		64,803		80,706
Net realized gains (losses) on investments		7,459		85,149		40,527		186,315
Net realized and unrealized gains (losses) on derivative instruments		(34,596)		(229,742)		(84,697)		(465,822)
Other Operating Income (Expense) [Member]
Segment Reporting Information [Line Items]
Net fee income and other		(12,200)		(13,200)		(23,700)		(24,200)
Corporate and Other
Segment Reporting Information [Line Items]
Gross premiums written	[13]	58,519		68,545		115,330		137,456
Net premiums written	[13]	2,824		3,508		6,134		6,672
Net premiums earned	[13]	2,824		3,508		6,134		6,672
Net losses and loss expenses	[1],[13]	7,595		5,482		14,886		10,419
Acquisition costs	[1],[13]	1,282		1,823		2,589		3,284
Operating expenses	[2],[13]	278		405		511		623
Underwriting profit (loss)	[13]	(6,331)		(4,202)		(11,852)		(7,654)
Net results from structured products	[13]	0	[3]	0	[4]	0	[5]	0	[6]
Net fee income and other	[13]	135	[7]	171	[8]	(303)	[9]	473	[10]
Net realized and unrealized gains (losses) on derivative instruments	[13]	(906)		906		(7,975)		(2,716)
Net income (loss) from investment fund affiliates and operating affiliates	[13]	73,522		34,597		125,392		42,668
Exchange (gains) losses	[13]	(5,643)		(19,100)		(8,979)		(52,919)
Corporate operating expenses	[13]	102,162		133,185		201,035		245,695
Contribution from P&C and Corporate and Other	[13]	(25,611)		(188,944)		(59,361)		(348,413)
Interest expense	[11],[13]	43,422		43,733		83,672		85,346
Non-controlling interests	[13]	8,423		5,044		69,429		66,187
Income tax	[13]	29,006		2,467		42,098		24,762
Corporate and Other | Excluding Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	[12],[13]	8,157		7,525		15,384		16,058
Net realized gains (losses) on investments	[13]	(7,971)		(8,480)		(8,584)		(5,736)
Corporate and Other | Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	[13]	31,439		39,146		64,803		80,706
Net realized gains (losses) on investments	[13]	7,459		85,220		40,527		186,386
Net realized and unrealized gains (losses) on derivative instruments	[13]	(34,596)		(229,742)		(84,697)		(465,822)
Total P&C | Net investment income
Segment Reporting Information [Line Items]
Net results from structured products		13,500		14,100		26,900		26,700
Total P&C | Interest Expense [Member]
Segment Reporting Information [Line Items]
Net results from structured products		(10,600)		(12,000)		(21,100)		(22,700)
Operating Segments [Member] | Total P&C
Segment Reporting Information [Line Items]
Gross premiums written		3,554,430		3,530,778		8,176,036		7,890,093
Net premiums written		2,646,441		2,722,611		5,626,201		5,784,206
Net premiums earned		2,514,093		2,528,704		5,033,574		4,880,150
Net losses and loss expenses	[1]	1,529,083		1,632,386		3,112,539		3,014,871
Acquisition costs	[1]	432,302		418,697		866,864		820,503
Operating expenses	[2]	358,116		375,728		715,602		767,394
Underwriting profit (loss)		194,592		101,893		338,569		277,382
Net results from structured products		2,923	[3]	2,059	[4]	5,837	[5]	4,029	[6]
Net fee income and other		(2,132)	[7]	(2,501)	[8]	521	[9]	(5,526)	[10]
Net realized and unrealized gains (losses) on derivative instruments		0		0		0
Contribution from P&C and Corporate and Other		$ 408,082		$ 283,970		$ 709,023		$ 590,375
Ratios - P&C operations
Loss and loss expense ratio	[14]	60.80%		64.60%		61.80%		61.80%
Underwriting expense ratio	[14]	31.50%		31.40%		31.50%		32.50%
Combined ratio	[14]	92.30%		96.00%		93.30%		94.30%
Operating Segments [Member] | Total P&C | Excluding Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	[12]	$ 155,559		$ 154,642		$ 302,125		$ 297,773
Net realized gains (losses) on investments		57,140		27,948		61,971		16,788
Operating Segments [Member] | Total P&C | Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net realized gains (losses) on investments		0		(71)		0		(71)
Operating Segments [Member] | Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		2,576,754		2,512,012		5,270,970		5,015,984
Net premiums written		1,741,201		1,780,192		3,249,792		3,284,126
Net premiums earned		1,652,304		1,696,720		3,287,619		3,290,594
Net losses and loss expenses	[1]	1,077,087		1,095,739		2,137,450		2,095,331
Acquisition costs	[1]	231,908		232,414		442,391		457,872
Operating expenses	[2]	288,681		307,468		574,410		619,900
Underwriting profit (loss)		54,628		61,099		133,368		117,491
Net results from structured products		1,981	[3]	2,765	[4]	4,250	[5]	3,987	[6]
Net fee income and other		$ (2,826)	[7]	$ (3,749)	[8]	$ (859)	[9]	$ (7,611)	[10]
Ratios - P&C operations
Loss and loss expense ratio	[14]	65.20%		64.60%		65.00%		63.70%
Underwriting expense ratio	[14]	31.50%		31.80%		30.90%		32.70%
Combined ratio	[14]	96.70%		96.40%		95.90%		96.40%
Operating Segments [Member] | Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		$ 977,676		$ 1,018,766		$ 2,905,066		$ 2,874,109
Net premiums written		905,240		942,419		2,376,409		2,500,080
Net premiums earned		861,789		831,984		1,745,955		1,589,556
Net losses and loss expenses	[1]	451,996		536,647		975,089		919,540
Acquisition costs	[1]	200,394		186,283		424,473		362,631
Operating expenses	[2]	69,435		68,260		141,192		147,494
Underwriting profit (loss)		139,964		40,794		205,201		159,891
Net results from structured products		942	[3]	(706)	[4]	1,587	[5]	42	[6]
Net fee income and other		$ 694	[7]	$ 1,248	[8]	$ 1,380	[9]	$ 2,085	[10]
Ratios - P&C operations
Loss and loss expense ratio	[14]	52.40%		64.50%		55.80%		57.80%
Underwriting expense ratio	[14]	31.40%		30.60%		32.40%		32.10%
Combined ratio	[14]	83.80%		95.10%		88.20%		89.90%

[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 $13.5 million and $10.6 million, respectively.

[4] The net results from P&C structured products include net investment income and interest expense of $14.1 million and $12.0 million, respectively.

[5] The net results from P&C structured products include net investment income and interest expense of $26.9 million and $21.1 million, respectively.

[6] The net results from P&C structured products include net investment income and interest expense of $26.7 million and $22.7 million, respectively.

[7] Net fee income and other includes operating expenses of $12.2 million from the Company's loss prevention consulting services business.

[8] Net fee income and other includes operating expenses of $13.2 million from the Company's loss prevention consulting services business.

[9] Net fee income and other includes operating expenses of $23.7 million from the Company's loss prevention consulting services business.

[10] Net fee income and other includes operating expenses of $24.2 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 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.