CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Cash flows from operating activities:
Net income	$ 103,514	$ 95,686	$ 176,719	$ 115,542
Adjustments to reconcile net income to net cash provided by operating activities:
Net investment losses			54,002	29,443
Net realized and unrealized gains on other investments			(26,876)	(40,226)
Amortization of fixed maturities			15,642	20,740
Interest in income (loss) of equity method investments			(1,891)	7,741
Amortization of value of business acquired	53,407	0	110,517	0
Other amortization and depreciation			22,680	12,638
Share-based compensation expense, net of cash payments			11,707	(10,846)
Non-cash foreign exchange losses			0	24,149	$ 24,000
Bargain purchase gain	0	(15,044)	0	(15,044)
Changes in:
Accrued interest receivable			(1,839)	4,208
Reinsurance recoverable balances			(283,733)	242,437
Deferred acquisition costs			(240,320)	(151,633)
Prepaid reinsurance premiums			(361,482)	(180,107)
Reserve for loss and loss expenses			118,803	8,015
Unearned premiums			991,992	724,407
Insurance and reinsurance balances, net			(654,090)	(540,557)
Other items			25,718	(102,014)
Net cash provided by (used in) operating activities			(42,451)	148,893
Cash flows from investing activities:
Fixed maturities			(4,910,021)	(4,766,037)
Equity securities			(49,498)	(101,289)
Mortgage loans			(60,195)	(10,207)
Other investments			(57,477)	(107,020)
Equity method investments			0	(1,000)
Short-term investments			(239,313)	(9,029)
Proceeds from the sale of:
Fixed maturities			4,452,631	3,955,065
Equity securities			219,916	127,182
Other investments			91,946	177,238
Short-term investments			101,510	13,539
Proceeds from redemption of fixed maturities			706,487	1,043,002
Proceeds from redemption of short-term investments			30,572	116,111
Proceeds from the repayment of mortgage loans			40,741	10,467
Purchase of other assets			(13,043)	(12,205)
Purchase of subsidiary, net			0	(78,872)
Net cash provided by investing activities			314,256	356,945
Cash flows from financing activities:
Repurchase of common shares - open market			0	(262,592)
Taxes paid on withholding shares			(8,676)	(24,370)
Dividends paid - common shares			(68,172)	(71,189)
Repurchase of preferred shares			0	(351,074)
Dividends paid - preferred shares			(21,313)	(31,532)
Net cash used in financing activities			(98,161)	(740,757)
Effect of exchange rate changes on foreign currency cash, cash equivalents, and restricted cash			(10,737)	8,682
Increase (decrease) in cash, cash equivalents, and restricted cash			162,907	(226,237)
Cash, cash equivalents, and restricted cash - beginning of period			1,363,786	1,241,507	1,241,507
Cash, cash equivalents, and restricted cash - end of period	$ 1,526,693	$ 1,015,270	1,526,693	1,015,270	$ 1,363,786
Supplemental disclosures of cash flow information:
Income taxes paid			7,350	4,104
Interest paid			$ 32,550	$ 24,438