Segment Information - Statement of earnings by segment (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018		Mar. 31, 2017
Segment Reporting Information [Line Items]
Gross premiums written		$ 4,979,735		$ 4,678,417
Net premiums written		3,155,718		2,983,070
Net premiums earned		2,600,288		2,522,791
Net losses and loss expenses	[1]	1,632,313		1,590,747
Acquisition costs		463,827		435,869	[1]
Operating expenses	[2]	390,060		357,719
Underwriting profit (loss)		114,088		138,456
Net results from structured products		3,411	[3]	2,914	[4]
Net fee income and other		(5,742)	[5]	2,215	[6]
Net realized and unrealized gains (losses) on derivative instruments		4,221		(7,069)
Net income (loss) from investment fund affiliates and operating affiliates		55,951		51,870
Foreign exchange (gains) losses		9,841		(3,336)
Corporate operating expenses		70,043		98,873
Contribution from P&C and Corporate and Other		237,781		267,191
Interest expense	[7]	44,646		40,250
Income tax		31,902		13,092
Non-controlling interests		8,585		61,006
Net income (loss) attributable to common shareholders		152,648		152,843
Excluding Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	[8]	175,772		153,793
Net realized investment gains (losses)		(60,843)		4,218
Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income		30,398		33,364
Net realized investment gains (losses)		(22,512)		33,068
Net realized and unrealized gains (losses) on derivative instruments		22,921		(50,101)
Corporate and Other
Segment Reporting Information [Line Items]
Gross premiums written	[9]	52,595		56,811
Net premiums written	[9]	3,107		3,310
Net premiums earned	[9]	3,107		3,310
Net losses and loss expenses	[1],[9]	10,307		7,291
Acquisition costs	[9]	1,011		1,307	[1]
Operating expenses	[2],[9]	141		233
Underwriting profit (loss)	[9]	(8,352)		(5,521)
Net results from structured products	[9]	0	[3]	0	[4]
Net fee income and other	[9]	69	[5]	(438)	[6]
Net realized and unrealized gains (losses) on derivative instruments	[9]	4,221		(7,069)
Net income (loss) from investment fund affiliates and operating affiliates	[9]	55,951		51,870
Foreign exchange (gains) losses	[9]	9,841		(3,336)
Corporate operating expenses	[9]	70,043		98,873
Contribution from P&C and Corporate and Other	[9]	10,445		(33,750)
Interest expense	[7],[9]	44,646		40,250
Income tax	[9]	31,902		13,092
Non-controlling interests	[9]	8,585		61,006
Corporate and Other | Excluding Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	[8],[9]	7,519		7,227
Net realized investment gains (losses)	[9]	114		(613)
Corporate and Other | Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	[9]	30,398		33,364
Net realized investment gains (losses)		(22,512)		33,068
Net realized and unrealized gains (losses) on derivative instruments	[9]	22,921		(50,101)
Operating segments | Total P&C
Segment Reporting Information [Line Items]
Gross premiums written		4,927,140		4,621,606
Net premiums written		3,152,611		2,979,760
Net premiums earned		2,597,181		2,519,481
Net losses and loss expenses	[1]	1,622,006		1,583,456
Acquisition costs		462,816		434,562	[1]
Operating expenses	[2]	389,919		357,486
Underwriting profit (loss)		122,440		143,977
Net results from structured products		3,411	[3]	2,914	[4]
Net fee income and other		(5,811)	[5]	2,653	[6]
Net realized and unrealized gains (losses) on derivative instruments		0
Contribution from P&C and Corporate and Other		$ 227,336		$ 300,941
Ratios - P&C operations
Loss and loss expense ratio	[10]	62.50%		62.80%
Underwriting expense ratio	[10]	32.80%		31.50%
Combined ratio	[10]	95.30%		94.30%
Operating segments | Total P&C | Excluding Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	[8]	$ 168,253		$ 146,566
Net realized investment gains (losses)		(60,957)		4,831
Operating segments | Total P&C | Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net realized investment gains (losses)		0		0
Operating segments | Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		2,866,539		2,694,216
Net premiums written		1,481,867		1,508,591
Net premiums earned		1,665,789		1,635,315
Net losses and loss expenses	[1]	1,064,992		1,060,363
Acquisition costs		221,962		210,483	[1]
Operating expenses	[2]	316,380		285,729
Underwriting profit (loss)		62,455		78,740
Net results from structured products		2,103	[3]	2,269	[4]
Net fee income and other		$ (6,703)	[5]	$ 1,967	[6]
Ratios - P&C operations
Loss and loss expense ratio	[10]	63.90%		64.80%
Underwriting expense ratio	[10]	32.40%		30.40%
Combined ratio	[10]	96.30%		95.20%
Operating segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		$ 2,060,601		$ 1,927,390
Net premiums written		1,670,744		1,471,169
Net premiums earned		931,392		884,166
Net losses and loss expenses	[1]	557,014		523,093
Acquisition costs		240,854		224,079	[1]
Operating expenses	[2]	73,539		71,757
Underwriting profit (loss)		59,985		65,237
Net results from structured products		1,308	[3]	645	[4]
Net fee income and other		$ 892	[5]	$ 686	[6]
Ratios - P&C operations
Loss and loss expense ratio	[10]	59.80%		59.20%
Underwriting expense ratio	[10]	33.80%		33.40%
Combined ratio	[10]	93.60%		92.60%

[1] The Company has reflected the amortization of certain fair value adjustments recorded in conjunction with the Company's acquisition of Catlin Group Ltd in 2015 ("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 $8.9 million, respectively.

[4] The net results from P&C structured products include net investment income and interest expense of $13.4 million and $10.5 million, respectively.

[5] Net fee income and other includes operating expenses of $12.5 million from the Company's loss prevention consulting services business.

[6] Net fee income and other includes operating expenses of $11.4 million from the Company's loss prevention consulting services business.

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

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

[9] 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.

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