Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CASH FROM OPERATING ACTIVITIES:
Net income to common stockholders	$ 549,094	$ 601,916	$ 503,694
Adjustments to reconcile net income to net cash from operating activities:
Net investment gains	(335,858)	(267,005)	(92,324)
Depreciation and amortization	112,956	86,051	85,139
Noncontrolling interests	4,243	1,569	413
Investment funds	(69,333)	(99,301)	(62,228)
Stock incentive plans	40,490	37,174	32,123
Change in:
Arbitrage trading account	(4,896)	(10,633)	(7,173)
Premiums and fees receivable	(67,752)	(60,403)	(60,942)
Reinsurance accounts	(66,542)	(235,455)	(31,930)
Deferred policy acquisition costs	30,343	(25,912)	(29,860)
Current income taxes	25,859	42,632	20,428
Deferred income taxes	(16,893)	9,012	47,260
Reserves for losses and loss expenses	438,530	572,196	397,685
Unearned premiums	4,160	149,683	142,699
Other	66,482	46,852	(63,680)
Net cash from operating activities	710,883	848,376	881,304
CASH FLOWS USED IN INVESTING ACTIVITIES:
Proceeds from sale of fixed maturity securities	4,035,162	2,440,310	1,388,680
Proceeds from sale of equity securities	195,270	143,042	15,833
Distributions from investment funds	247,404	142,601	177,424
Proceeds from maturities and prepayments of fixed maturity securities	3,556,744	2,189,365	2,999,339
Purchase of fixed maturity securities	(7,940,957)	(5,541,202)	(4,455,223)
Purchase of equity securities	(27,522)	(202,736)	(29,526)
Real estate purchased	(236,039)	(299,123)	(222,659)
Change in loans receivable	27,135	166,327	48,909
Net additions to property, furniture and equipment	(115,719)	(50,829)	(63,562)
Change in balances due from security brokers	(4,372)	20,992	(22,666)
Proceeds from Divestiture of Businesses, Net of Cash Divested	0	250,216	0
Payment for business purchased, net of cash acquired	(70,570)	(53,451)	(7,312)
Net cash used in investing activities	(333,464)	(794,488)	(170,763)
CASH FLOWS USED IN FINANCING ACTIVITIES:
Net proceeds from issuance of debt	6,983	388,769	9,056
Repayment of senior notes and other debt	(20)	(75,487)	(281,086)
Cash dividends to common stockholders	(188,199)	(183,999)	(58,034)
Purchase of common treasury shares	(47,807)	(132,392)	(223,652)
Other, net	(6,043)	(3,823)	(1,602)
Net cash used in financing activities	(235,086)	(6,932)	(555,318)
Net impact on cash due to change in foreign exchange rates	12,853	(15,302)	(66,033)
Net increase in cash and cash equivalents	155,186	31,654	89,190
Cash and cash equivalents at beginning of year	795,285	763,631	674,441
Cash and cash equivalents at end of year	$ 950,471	$ 795,285	$ 763,631