Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement of Cash Flows [Abstract]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 937,428	$ 956,603
CASH FROM (USED IN) OPERATING ACTIVITIES:
Net income to common stockholders	180,722	166,396
Adjustments to reconcile net income to net cash from (used in) operating activities:
Net realized and unrealized gains on investments	(68,653)	(48,464)
Depreciation and amortization	27,424	29,027
Noncontrolling interests	892	1,177
Investment funds	(11,411)	(40,354)
Stock incentive plans	14,828	9,434
Change in:
Arbitrage trading account	(7,753)	497
Premiums and fees receivable	(156,450)	(74,492)
Reinsurance accounts	(20,272)	(100,379)
Deferred policy acquisition costs	(17,732)	(6,851)
Income taxes	36,180	50,505
Reserves for losses and loss expenses	119,780	120,063
Unearned premiums	135,184	116,627
Other	(154,409)	(243,221)
Net cash from (used in) operating activities	78,330	(20,035)
CASH FROM (USED IN) INVESTING ACTIVITIES:
Proceeds from sale of fixed maturity securities	973,626	2,004,008
Proceeds from sale of equity securities	37,762	152,949
(Contributions to) distributions from investment funds	(13,874)	33,695
Proceeds from maturities and prepayments of fixed maturity securities	686,795	97,911
Purchase of fixed maturity securities	(1,528,267)	(2,085,683)
Purchase of equity securities	(35,314)	(44,219)
Real estate purchased	(52,901)	(336,601)
Change in loans receivable	(726)	2,058
Net additions to property, furniture and equipment	(3,299)	(14,102)
Change in balances due to security brokers	(22,011)	58,500
Net cash from (used in) investing activities	41,791	(131,484)
CASH (USED IN) FROM FINANCING ACTIVITIES:
Repayment of senior notes and other debt	0	(23)
Net proceeds from issuance of debt	0	181,792
Cash dividends to common stockholders	0	(17,016)
Purchase of common treasury shares	0	(6,799)
Other, net	(976)	(544)
Net cash (used in) from financing activities	(976)	157,410
Net impact on cash due to change in foreign exchange rates	681	241
Net change in cash and cash equivalents	119,826	$ 6,132
Cash and cash equivalents at beginning of year	817,602
Cash and cash equivalents at end of period	$ 937,428