Segment Information - Statement of earnings by segment (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2017		Mar. 31, 2016
Segment Reporting Information [Line Items]
Gross premiums written		$ 4,678,417		$ 4,428,226
Net premiums written		2,983,070		3,064,759
Net premiums earned		2,522,791		2,354,610
Net losses and loss expenses	[1]	1,590,747		1,387,422
Acquisition costs		435,869		403,267
Operating expenses	[2]	357,719		391,884
Underwriting profit (loss)		138,456		172,037
Net results from structured products		2,914	[3]	1,970	[4]
Net fee income and other	[5]	2,215		(2,723)
Net realized gains (losses) on investments		37,286		92,750
Net realized and unrealized gains (losses) on derivative instruments		(7,069)		(3,622)
Net income (loss) from investment fund affiliates and operating affiliates		51,870		8,071
Exchange (gains) losses		(3,336)		(33,819)
Corporate operating expenses		98,873		112,510
Contribution from P&C and Corporate and Other		267,191		146,936
Interest expense	[6]	40,250		41,613
Non-controlling interests		61,006		61,143
Income tax		13,092		22,295
Net income (loss) attributable to common shareholders		152,843		21,885
Corporate and Other
Segment Reporting Information [Line Items]
Gross premiums written	[7]	56,811		68,911
Net premiums written	[7]	3,310		3,164
Net premiums earned	[7]	3,310		3,164
Net losses and loss expenses	[1],[7]	7,291		4,937
Acquisition costs	[1],[7]	1,307		1,461
Operating expenses	[2],[7]	233		218
Underwriting profit (loss)	[7]	(5,521)		(3,452)
Net results from structured products	[7]	0	[3]	0	[4]
Net fee income and other	[5],[7]	(438)		302
Net realized and unrealized gains (losses) on derivative instruments	[7]	(7,069)		(3,622)
Net income (loss) from investment fund affiliates and operating affiliates	[7]	51,870		8,071
Exchange (gains) losses	[7]	(3,336)		(33,819)
Corporate operating expenses	[7]	98,873		112,510
Contribution from P&C and Corporate and Other	[7]	(33,750)		(159,469)
Interest expense	[6],[7]	40,250		41,613
Non-controlling interests	[7]	61,006		61,143
Income tax	[7]	13,092		22,295
Excluding Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	[8]	153,793		151,664
Net realized gains (losses) on investments		4,218		(8,416)
Excluding Life Funds Withheld Assets | Corporate and Other
Segment Reporting Information [Line Items]
Net investment income	[7],[8]	7,227		8,533
Net realized gains (losses) on investments	[7]	(613)		2,744
Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income		33,364		41,560
Net realized gains (losses) on investments		33,068		101,166
Net realized and unrealized gains (losses) on derivative instruments		(50,101)		(236,080)
Life Funds Withheld Assets | Corporate and Other
Segment Reporting Information [Line Items]
Net investment income	[7]	33,364		41,560
Net realized gains (losses) on investments	[7]	33,068		101,166
Net realized and unrealized gains (losses) on derivative instruments	[7]	(50,101)		(236,080)
Operating Segments [Member] | Total P&C
Segment Reporting Information [Line Items]
Gross premiums written		4,621,606		4,359,315
Net premiums written		2,979,760		3,061,595
Net premiums earned		2,519,481		2,351,446
Net losses and loss expenses	[1]	1,583,456		1,382,485
Acquisition costs	[1]	434,562		401,806
Operating expenses	[2]	357,486		391,666
Underwriting profit (loss)		143,977		175,489
Net results from structured products		2,914	[3]	1,970	[4]
Net fee income and other	[5]	2,653		(3,025)
Net realized and unrealized gains (losses) on derivative instruments		0
Contribution from P&C and Corporate and Other		$ 300,941		$ 306,405
Ratios - P&C operations
Loss and loss expense ratio	[9]	62.80%		58.80%
Underwriting expense ratio	[9]	31.50%		33.70%
Combined ratio	[9]	94.30%		92.50%
Operating Segments [Member] | Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		$ 2,694,216		$ 2,503,972
Net premiums written		1,508,591		1,503,934
Net premiums earned		1,635,315		1,593,874
Net losses and loss expenses	[1]	1,060,363		999,592
Acquisition costs	[1]	210,483		225,458
Operating expenses	[2]	285,729		312,432
Underwriting profit (loss)		78,740		56,392
Net results from structured products		2,269	[3]	1,222	[4]
Net fee income and other	[5]	$ 1,967		$ (3,862)
Ratios - P&C operations
Loss and loss expense ratio	[9]	64.80%		62.70%
Underwriting expense ratio	[9]	30.40%		33.80%
Combined ratio	[9]	95.20%		96.50%
Operating Segments [Member] | Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		$ 1,927,390		$ 1,855,343
Net premiums written		1,471,169		1,557,661
Net premiums earned		884,166		757,572
Net losses and loss expenses	[1]	523,093		382,893
Acquisition costs	[1]	224,079		176,348
Operating expenses	[2]	71,757		79,234
Underwriting profit (loss)		65,237		119,097
Net results from structured products		645	[3]	748	[4]
Net fee income and other	[5]	$ 686		$ 837
Ratios - P&C operations
Loss and loss expense ratio	[9]	59.20%		50.50%
Underwriting expense ratio	[9]	33.40%		33.80%
Combined ratio	[9]	92.60%		84.30%
Operating Segments [Member] | Excluding Life Funds Withheld Assets | Total P&C
Segment Reporting Information [Line Items]
Net investment income	[8]	$ 146,566		$ 143,131
Net realized gains (losses) on investments		4,831		$ (11,160)
Operating Segments [Member] | Life Funds Withheld Assets | Total P&C
Segment Reporting Information [Line Items]
Net realized gains (losses) on investments		$ 0

[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 $13.4 million and $10.5 million, respectively.

[4] The net results from P&C structured products include net investment income and interest expense of $12.7 million and $10.7 million, respectively.

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

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

[7] 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.

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

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