Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities [Abstract]
Net income	$ 606,011	$ 121,088	$ 308,253
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Net (gains) losses on investments and other financial instruments	42,476	(3,621)	(30,751)
Loss on induced conversion and debt extinguishment	0	51,469	75,075
Deferred income tax provision	120,573	166,527	170,887
Impairment of goodwill	0	184,374	0
Amortization and impairment of other acquired intangible assets	12,429	27,797	13,221
Depreciation, other amortization, and other impairments, net	56,661	58,038	57,795
Change in:
Accounts and notes receivable	(4,599)	3,628	(16,011)
Prepaid reinsurance premiums	(31,119)	(157,071)	(188,947)
Unearned premiums	15,419	42,716	862
Reserve for losses and LAE	(109,642)	(252,681)	(216,135)
Reinsurance funds withheld	32,814	130,397	158,001
Other assets	43,562	(16,491)	(7,662)
Other liabilities	(106,799)	4,405	57,136
Net cash provided by (used in) operating activities	677,786	360,575	381,724
Cash flows from investing activities:
Proceeds from sales of Fixed-maturity investments available for sale	728,584	888,219	687,173
Proceeds from sales of Trading securities	58,317	194,784	290,855
Proceeds from sales of Equity securities	95,697	38,318	74,868
Proceeds from redemptions of Fixed-maturity investments available for sale	457,595	463,548	337,630
Proceeds from redemptions of Trading securities	54,329	79,296	123,645
Purchases of Fixed-maturity investments available for sale	(1,875,069)	(1,947,916)	(1,990,652)
Purchases of Equity securities	(69,160)	(213,469)	(830)
Sales, redemptions and (purchases) of Short-term investments, net	(108,325)	324,258	334,456
Sales, redemptions and (purchases) of Other assets and other invested assets, net	2,590	882	2,489
Net Cash Received (Transferred) In Sale Of Subsidiaries	0	(650)	0
Purchases of property and equipment, net	(26,008)	(28,676)	(35,542)
Acquisitions, net of cash acquired	(7,964)	(86)	(150)
Net cash provided by (used in) investing activities	(689,414)	(201,492)	(176,058)
Cash flows from financing activities:
Dividends paid	(2,140)	(2,154)	(2,105)
Issuance of senior notes, net	0	442,163	343,417
Purchases and redemptions of senior notes	0	(593,527)	(445,072)
Proceeds from termination of capped calls	0	4,208	0
Issuance of common stock	1,385	7,132	717
Purchase of common shares	(50,053)	(6)	(100,188)
Credit facility commitment fees paid	(1,510)	(1,993)	0
Change in secured borrowings	39,342	19,357	0
Proceeds from secured borrowings (with terms greater than 3 months)	56,449	0	0
Payments of secured borrowings (with terms greater than 3 months)	(20,917)	0	0
Excess tax benefits from stock-based awards	0	0	333
Repayment of other borrowings	(170)	(264)	(371)
Net cash provided by (used in) financing activities	22,386	(125,084)	(203,269)
Effect of exchange rate changes on cash and restricted cash	0	431	(481)
Increase (decrease) in cash and restricted cash	10,758	34,430	1,916
Cash and restricted cash, beginning of period	96,244	61,814	59,898
Cash and restricted cash, end of period	107,002	96,244	61,814
Supplemental disclosures of cash flow information:
Income taxes paid (received)	8,364	94,328	(673)
Interest paid	$ 56,688	$ 57,453	$ 65,531