Consolidated Statements Of Changes In Equity - USD ($) $ in Thousands	Total	Total Equity [Member]	Total Shareholders' Equity [Member]	Common Stock [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income [Member]	Noncontrolling Interests [Member]	Redeemable Noncontrolling Interests [Member]	Brahmin [Member] Redeemable Noncontrolling Interests [Member]	Nephila Holdings Limited [Member] Total Equity [Member]	Nephila Holdings Limited [Member] Noncontrolling Interests [Member]
Balance at Dec. 31, 2015		$ 7,840,609	$ 7,834,150	$ 3,342,357	$ 3,137,285	$ 1,354,508	$ 6,459
Balance, shares at Dec. 31, 2015				13,959,000
Balance, redeemable noncontrolling interests at Dec. 31, 2015								$ 62,958
Net income (loss)	$ 460,443	455,788	455,689		455,689	0	99	4,655
Other comprehensive income (loss)	211,364	211,358	211,358		0	211,358	0	6
Comprehensive Income (Loss)	671,807	667,146	667,047				99	4,661
Issuance of common stock		4,623	4,623	$ 4,623	0	0	0	0
Issuance of common stock, shares				54,000
Repurchase of common stock		(51,142)	(51,142)	$ 0	(51,142)	0	0	0
Repurchase of common stock, shares				(58,000)
Restricted stock awards expensed		21,336	21,336	$ 21,336	0	0	0	0
Restricted stock awards expensed, shares				0
Adjustment of redeemable noncontrolling interests		(15,472)	(15,472)	$ 0	(15,472)	0	0	15,472
Adjustment of redeemable noncontrolling interests, shares				0
Purchase of noncontrolling interest		(350)	(350)	$ (350)	0	0	0	3,517
Purchase of noncontrolling interest, shares				0
Other		(39)	35	$ 0	35	0	(74)	(5,896)
Other, shares				0
Balance at Dec. 31, 2016		8,467,411	8,460,927	$ 3,368,666	3,526,395	1,565,866	6,484
Balance, shares at Dec. 31, 2016				13,955,000
Balance, redeemable noncontrolling interests at Dec. 31, 2016								73,678
Net income (loss)	400,758	394,374	395,269		395,269	0	(895)	6,384
Other comprehensive income (loss)	779,751	779,705	779,705		0	779,705	0	46
Comprehensive Income (Loss)	1,180,509	1,174,079	1,174,974				(895)	6,430
Issuance of common stock		552	552	$ 552	0	0	0	0
Issuance of common stock, shares				58,000
Repurchase of common stock		110,838	110,838	$ 0	110,838	0	0	0
Repurchase of common stock, shares				109,000
Restricted stock awards expensed		15,881	15,881	$ 15,881	0	0	0	0
Restricted stock awards expensed, shares				0
Acquisitions		0	0	$ 0	0	0	0	66,600
Acquisition, shares				0
Adjustment of redeemable noncontrolling interests		(33,738)	(33,738)	$ 0	(33,738)	0	0	33,738
Adjustment of redeemable noncontrolling interests, shares				0
Purchase of noncontrolling interest		(11,285)	(2,955)	$ (2,955)	0	0	(8,330)	(6,179)
Purchase of noncontrolling interest, shares				0
Other		(481)	(655)	$ (310)	(345)	0	174	(7,998)
Other, shares				0
Balance at Dec. 31, 2017	9,501,581	9,501,581	9,504,148	$ 3,381,834	3,776,743	2,345,571	(2,567)
Balance, shares at Dec. 31, 2017				13,904,000
Balance, redeemable noncontrolling interests at Dec. 31, 2017	166,269							166,269
Net income (loss)	(130,353)	(129,355)	(128,180)		(128,180)	0	(1,175)	(998)
Other comprehensive income (loss)	(247,630)	(247,590)	(247,590)		0	(247,590)	0	(40)
Comprehensive Income (Loss)	(377,983)	(376,945)	(375,770)				(1,175)	(1,038)
Issuance of common stock		2	2	$ 2	0	0	0	0
Issuance of common stock, shares				32,000
Repurchase of common stock	$ (39,000)	(54,007)	(54,007)	$ 0	(54,007)	0	0	0
Repurchase of common stock, shares	(35,174)			(48,000)
Restricted stock awards expensed		16,191	16,191	$ 16,191	0	0	0	0
Restricted stock awards expensed, shares				0
Acquisitions		0	0	$ 0	0	0	0	0	$ 19,670	$ 23,392	$ 23,392
Acquisition, shares				0
Adjustment of redeemable noncontrolling interests		(4,828)	(4,828)	$ 0	(4,828)	0	0	4,828
Adjustment of redeemable noncontrolling interests, shares				0
Purchase of noncontrolling interest		(4,986)	(4,986)	$ (4,986)	0	0	0	(7,104)
Purchase of noncontrolling interest, shares				0
Other		(423)	(422)	$ (48)	(374)	0	(1)	(8,563)
Other, shares				0
Balance at Dec. 31, 2018	$ 9,100,302	$ 9,100,302	$ 9,080,653	$ 3,392,993	$ 5,782,310	$ (94,650)	$ 19,649
Balance, shares at Dec. 31, 2018				13,888,000
Balance, redeemable noncontrolling interests at Dec. 31, 2018	$ 174,062							$ 174,062