Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows provided by (used in) operating activities
Net (loss) income	$ (404,902)	$ 487,202	$ 467,857
Adjustments to reconcile net (loss) income to cash provided by (used in) operating activities:
Share compensation expenses	40,111	42,907	38,341
Loss on redemption of AlphaCat ILS fund	402	0	0
Amortization of discount on senior notes	108	108	108
(Income) loss from investment and operating affiliates	(22,010)	2,106	(332)
Net realized and change in net unrealized (gains) losses on investments	(10,838)	(32,628)	30,097
Amortization of intangible assets	8,261	5,666	5,666
Foreign exchange (gains) losses included in net income	(14,719)	3,043	18,907
Amortization of premium on fixed maturity investments	15,910	17,961	23,075
Change in operational balance sheet items:
Premiums receivable	353,787	(76,659)	44,154
Deferred acquisition costs	(8,705)	(28,225)	(19,980)
Prepaid reinsurance premiums	172,215	(4)	4,955
Loss reserves recoverable	(745,979)	(85,264)	24,659
Paid losses recoverable	6,352	(11,932)	15,111
Reserve for losses and loss expenses	1,717,794	37,042	(231,345)
Unearned premiums	(335,512)	109,839	(23,019)
Reinsurance balances payable	(18,381)	(18,487)	(52,785)
Other operational balance sheet items, net	(364,160)	(42,472)	(10,963)
Net cash provided by operating activities	389,734	410,203	334,506
Cash flows provided by (used in) investing activities
Proceeds on sales of fixed maturity investments	3,135,835	2,671,226	3,842,408
Proceeds on maturities of fixed maturity investments	482,006	349,722	332,410
Purchases of fixed maturity investments	(3,850,224)	(3,074,703)	(4,225,031)
Purchases of short-term investments, net	(577,391)	(856,021)	(436,690)
Purchases of other investments, net	(7,093)	(64,943)	(5,988)
Decrease (increase) in securities lending collateral	7,062	(4,916)	(4,393)
Distributions from operating affiliates	0	369	46,603
Distributions (investments) in investment affiliates, net	22,304	(14,841)	(19,886)
Purchase of subsidiary, net of cash	(183,923)	0	0
Net cash used in investing activities	(971,424)	(994,107)	(470,567)
Cash flows provided by (used in) financing activities
Net proceeds on issuance of notes payable to AlphaCat investors	311,054	296,325	75,493
Proceeds drawn on credit facilities	206,000	0	0
Net proceeds on issuance of preferred shares	241,686	144,852	0
(Redemption) issuance of common shares, net	(11,578)	(7,701)	17,407
Purchases of common shares under share repurchase program	(26,568)	(212,675)	(260,430)
Dividends paid on preferred shares	(15,861)	(4,455)	0
Dividends paid on common shares	(123,859)	(115,625)	(112,967)
(Decrease) increase in securities lending payable	(7,528)	4,916	4,393
Third party investments in redeemable noncontrolling interests	397,199	393,450	499,200
Third party redemptions of redeemable noncontrolling interests	(258,869)	(17,285)	(86,933)
Third party investments in noncontrolling interests	258,300	171,674	9,600
Third party distributions of noncontrolling interests	(200,690)	(127,103)	(168,758)
Third party subscriptions received (deployed) on funds and sidecars, net	458,881	(225,284)	249,636
Net cash provided by financing activities	1,228,167	301,089	226,641
Effect of foreign currency rate changes on cash and cash equivalents and restricted cash	12,244	(22,632)	(17,605)
Net increase (decrease) in cash and cash equivalents and restricted cash	658,721	(305,447)	72,975
Cash and cash equivalents and restricted cash - beginning of year	490,932	796,379	723,404
Cash and cash equivalents and restricted cash - end of year	1,149,653	490,932	796,379
Supplemental disclosure of cash flow information:
Taxes paid during the year	4,321	6,034	16,050
Interest paid during the year	$ 54,433	$ 54,638	$ 55,047