Segment Information - Statement of earnings by segment (Details) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended
		Jun. 30, 2018		Jun. 30, 2017		Jun. 30, 2018		Jun. 30, 2017
Segment Reporting Information [Line Items]
Gross premiums written		$ 3,985,366		$ 3,612,949		$ 8,965,101		$ 8,291,366
Net premiums written		2,821,744		2,649,265		5,977,462		5,632,335
Net premiums earned		2,695,169		2,516,917		5,295,457		5,039,708
Net losses and loss expenses	[1]	1,731,426		1,536,678		3,363,739		3,127,425
Acquisition costs		471,491		433,584		935,318		869,453	[1]
Operating expenses	[2]	387,089		358,394		777,149		716,113
Underwriting profit (loss)		105,163		188,261		219,251		326,717
Net results from structured products		8,382	[3]	2,923	[4]	11,793	[5]	5,837	[6]
Net fee income and other		(3,806)	[7]	(1,997)	[8]	(9,548)	[9]	218	[10]
Net realized and unrealized gains (losses) on derivative instruments		16,126		(906)		20,347		(7,975)
Net income (loss) from investment fund affiliates and operating affiliates		67,735		73,522		123,686		125,392
Foreign exchange (gains) losses		(12,140)		(5,643)		(2,299)		(8,979)
Corporate operating expenses		71,308		102,162		141,351		201,035
Contribution from P&C and Corporate and Other		408,605		382,471		646,386		649,662
Interest expense	[11]	44,769		43,422		89,415		83,672
Income tax		27,036		29,006		58,938		42,098
Non-controlling interests		17,804		8,423		26,389		69,429
Net income (loss) attributable to common shareholders		318,996		301,620		471,644		454,463
Excluding Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	[12]	189,236		163,716		365,008		317,509
Net realized investment gains (losses)		27,396		49,169		(33,447)		53,387
Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income		30,252		31,439		60,650		64,803
Net realized investment gains (losses)		63,409		7,459		40,897		40,527
Net realized and unrealized gains (losses) on derivative instruments		(36,120)		(34,596)		(13,199)		(84,697)
Corporate and Other
Segment Reporting Information [Line Items]
Gross premiums written	[13]	55,093		58,519		107,688		115,330
Net premiums written	[13]	2,505		2,824		5,612		6,134
Net premiums earned	[13]	2,505		2,824		5,612		6,134
Net losses and loss expenses	[1],[13]	10,117		7,595		20,424		14,886
Acquisition costs	[13]	1,259		1,282		2,270		2,589	[1]
Operating expenses	[2],[13]	195		278		336		511
Underwriting profit (loss)	[13]	(9,066)		(6,331)		(17,418)		(11,852)
Net results from structured products	[13]	0	[3]	0	[4]	0	[5]	0	[6]
Net fee income and other	[13]	45	[7]	135	[8]	114	[9]	(303)	[10]
Net realized and unrealized gains (losses) on derivative instruments	[13]	16,126		(906)		20,347		(7,975)
Net income (loss) from investment fund affiliates and operating affiliates	[13]	67,735		73,522		123,686		125,392
Foreign exchange (gains) losses	[13]	(12,140)		(5,643)		(2,299)		(8,979)
Corporate operating expenses	[13]	71,308		102,162		141,351		201,035
Contribution from P&C and Corporate and Other	[13]	82,934		(25,611)		93,379		(59,361)
Interest expense	[11],[13]	44,769		43,422		89,415		83,672
Income tax	[13]	27,036		29,006		58,938		42,098
Non-controlling interests	[13]	17,804		8,423		26,389		69,429
Corporate and Other | Excluding Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	[12],[13]	7,227		8,157		14,746		15,384
Net realized investment gains (losses)	[13]	2,494		(7,971)		2,608		(8,584)
Corporate and Other | Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	[13]	30,252		31,439		60,650		64,803
Net realized investment gains (losses)		63,409		7,459		40,897		40,527
Net realized and unrealized gains (losses) on derivative instruments	[13]	(36,120)		(34,596)		(13,199)		(84,697)
Operating segments | Total P&C
Segment Reporting Information [Line Items]
Gross premiums written		3,930,273		3,554,430		8,857,413		8,176,036
Net premiums written		2,819,239		2,646,441		5,971,850		5,626,201
Net premiums earned		2,692,664		2,514,093		5,289,845		5,033,574
Net losses and loss expenses	[1]	1,721,309		1,529,083		3,343,315		3,112,539
Acquisition costs		470,232		432,302		933,048		866,864	[1]
Operating expenses	[2]	386,894		358,116		776,813		715,602
Underwriting profit (loss)		114,229		194,592		236,669		338,569
Net results from structured products		8,382	[3]	2,923	[4]	11,793	[5]	5,837	[6]
Net fee income and other		(3,851)	[7]	(2,132)	[8]	(9,662)	[9]	521	[10]
Net realized and unrealized gains (losses) on derivative instruments		0		0		0
Contribution from P&C and Corporate and Other		$ 325,671		$ 408,082		$ 553,007		$ 709,023
Ratios - P&C operations
Loss and loss expense ratio	[14]	63.90%		60.80%		63.20%		61.80%
Underwriting expense ratio	[14]	31.90%		31.50%		32.30%		31.50%
Combined ratio	[14]	95.80%		92.30%		95.50%		93.30%
Operating segments | Total P&C | Excluding Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	[12]	$ 182,009		$ 155,559		$ 350,262		$ 302,125
Net realized investment gains (losses)		24,902		57,140		(36,055)		61,971
Operating segments | Total P&C | Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net realized investment gains (losses)		0		0		0		0
Operating segments | Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		2,881,921		2,576,754		5,748,460		5,270,970
Net premiums written		1,929,564		1,741,201		3,411,431		3,249,792
Net premiums earned		1,748,355		1,652,304		3,414,144		3,287,619
Net losses and loss expenses	[1]	1,224,756		1,077,087		2,289,748		2,137,450
Acquisition costs		237,694		231,908		459,656		442,391	[1]
Operating expenses	[2]	306,585		288,681		622,965		574,410
Underwriting profit (loss)		(20,680)		54,628		41,775		133,368
Net results from structured products		1,810	[3]	1,981	[4]	3,913	[5]	4,250	[6]
Net fee income and other		$ (4,991)	[7]	$ (2,826)	[8]	$ (11,694)	[9]	$ (859)	[10]
Ratios - P&C operations
Loss and loss expense ratio	[14]	70.10%		65.20%		67.10%		65.00%
Underwriting expense ratio	[14]	31.10%		31.50%		31.70%		30.90%
Combined ratio	[14]	101.20%		96.70%		98.80%		95.90%
Operating segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		$ 1,048,352		$ 977,676		$ 3,108,953		$ 2,905,066
Net premiums written		889,675		905,240		2,560,419		2,376,409
Net premiums earned		944,309		861,789		1,875,701		1,745,955
Net losses and loss expenses	[1]	496,553		451,996		1,053,567		975,089
Acquisition costs		232,538		200,394		473,392		424,473	[1]
Operating expenses	[2]	80,309		69,435		153,848		141,192
Underwriting profit (loss)		134,909		139,964		194,894		205,201
Net results from structured products		6,572	[3]	942	[4]	7,880	[5]	1,587	[6]
Net fee income and other		$ 1,140	[7]	$ 694	[8]	$ 2,032	[9]	$ 1,380	[10]
Ratios - P&C operations
Loss and loss expense ratio	[14]	52.60%		52.40%		56.20%		55.80%
Underwriting expense ratio	[14]	33.10%		31.40%		33.40%		32.40%
Combined ratio	[14]	85.70%		83.80%		89.60%		88.20%

[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 $3.9 million, respectively.

[4] The net results from P&C structured products include net investment income and interest expense of $13.5 million and $10.6 million, respectively.

[5] The net results from P&C structured products include net investment income and interest expense of $24.6 million and $12.8 million, respectively.

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

[7] Net fee income and other includes operating expenses of $12.1 million from the Company's loss prevention consulting services business.

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

[9] Net fee income and other includes operating expenses of $24.6 million from the Company's loss prevention consulting services business.

[10] Net fee income and other includes operating expenses of $23.7 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.