Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows provided by (used in) operating activities
Net income	$ 487,202	$ 467,857	$ 554,843
Adjustments to reconcile net income to cash provided by operating activities:
Share compensation expenses	42,907	38,341	33,073
Gain on sale of subsidiary	0	0	(709)
Amortization of discount on senior notes	108	108	108
Loss (income) from investment and operating affiliates	2,106	(332)	(4,071)
Net realized and change in net unrealized (gains) losses on investments	(32,628)	30,097	(12,075)
Amortization of intangible assets	5,666	5,666	4,538
Foreign exchange losses included in net income	3,043	18,907	23,591
Amortization of premium on fixed maturity investments	17,961	23,075	19,107
Change in operational balance sheet items:
Premiums receivable	(76,659)	44,154	22,034
Deferred acquisition costs	(28,225)	(19,980)	(27,131)
Prepaid reinsurance premiums	(4)	4,955	34,443
Loss reserves recoverable	(85,264)	24,659	81,824
Paid losses recoverable	(11,932)	15,111	43,580
Reserve for losses and loss expenses	37,042	(231,345)	(404,280)
Unearned premiums	109,839	(23,019)	18,161
Reinsurance balances payable	(18,487)	(52,785)	(32,262)
Other operational balance sheet items, net	(42,472)	(10,963)	(14,613)
Net cash provided by operating activities	410,203	334,506	340,161
Cash flows provided by (used in) investing activities
Proceeds on sales of fixed maturity investments	2,671,226	3,842,408	4,756,555
Proceeds on maturities of fixed maturity investments	349,722	332,410	554,619
Purchases of fixed maturity investments	(3,074,703)	(4,225,031)	(4,391,814)
Purchases of short-term investments, net	(856,021)	(436,690)	(329,301)
Purchases of other investments, net	(64,943)	(5,988)	(143,597)
(Increase) decrease in securities lending collateral	(4,916)	(4,393)	2,922
Distributions from operating affiliates	369	46,603	0
Investments in investment affiliates, net	(14,841)	(19,886)	(12,468)
Decrease in restricted cash	2,314	99,733	27,489
Proceeds on sale of subsidiary, net of cash	0	0	16,459
Purchase of subsidiary, net of cash	0	0	(643,545)
Net cash used in investing activities	(991,793)	(370,834)	(162,681)
Cash flows provided by (used in) financing activities
Net proceeds on issuance of notes payable to AlphaCat investors	296,325	75,493	0
Net proceeds on issuance of preferred shares	144,852	0	0
(Redemption) issuance of common shares, net	(7,701)	17,407	5,347
Purchases of common shares under share repurchase program	(212,675)	(260,430)	(510,952)
Dividends paid on preferred shares	(4,455)	0	0
Dividends paid on common shares	(115,625)	(112,967)	(117,043)
Increase (decrease) in securities lending payable	4,916	4,393	(2,922)
Third party investments in redeemable noncontrolling interests	393,450	499,200	300,200
Third party redemptions of redeemable noncontrolling interests	(17,285)	(86,933)	(13,192)
Third party investments in noncontrolling interests	171,674	9,600	117,243
Third party distributions of noncontrolling interests	(127,103)	(168,758)	(237,183)
Third party subscriptions (deployed) received on AlphaCat Funds and Sidecars	(225,284)	249,636	127,400
Net cash provided by (used in) financing activities	301,089	226,641	(331,102)
Effect of foreign currency rate changes on cash and cash equivalents	(22,632)	(17,605)	(25,310)
Net (decrease) increase in cash and cash equivalents	(303,133)	172,708	(178,932)
Cash and cash equivalents - beginning of year	723,109	550,401	729,333
Cash and cash equivalents - end of year	419,976	723,109	550,401
Supplemental disclosure of cash flow information:
Taxes paid during the year	6,034	16,050	42,689
Interest paid during the year	$ 54,638	$ 55,047	$ 54,526