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 (Loss) [Member]	Noncontrolling Interests [Member]	Redeemable Noncontrolling Interests [Member]	Nephila Holdings Limited [Member] Total Equity [Member]	Nephila Holdings Limited [Member] Noncontrolling Interests [Member]	Nephila Holdings Limited [Member] Redeemable Noncontrolling Interests [Member]
Balance at Dec. 31, 2017		$ 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
Net income (loss)	$ 623,021	622,094	623,363		623,363	0	(1,269)	927
Total other comprehensive income (loss)	(318,800)	(318,760)	(318,760)		0	(318,760)	0	(40)
Comprehensive Income (Loss)	304,221	303,334	304,603				(1,269)	887
Issuance of common stock		2	2	$ 2	0	0	0	0
Issuance of common stock, shares				8,000
Repurchase of common stock		(30,829)	(30,829)	$ 0	(30,829)	0	0	0
Repurchase of common stock, shares				(28,000)
Restricted stock units expensed		15,521	15,521	$ 15,521	0	0	0	0
Restricted stock units expensed, shares				0
Adjustment of redeemable noncontrolling interests		(6,621)	(6,621)	$ 0	(6,621)	0	0	6,621
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		1,827	1,829	$ (49)	1,878	0	(2)	(7,934)
Other, shares				0
Balance at Sep. 30, 2018		9,780,154	9,783,992	$ 3,392,322	6,576,562	(184,892)	(3,838)
Balance, shares at Sep. 30, 2018				13,884,000
Balance, redeemable noncontrolling interests at Sep. 30, 2018								158,739
Balance at Jun. 30, 2018		9,477,414	9,480,860	$ 3,387,121	6,184,299	(90,560)	(3,446)
Balance, shares at Jun. 30, 2018				13,886,000
Balance, redeemable noncontrolling interests at Jun. 30, 2018								155,998
Net income (loss)	409,028	409,079	409,438		409,438	0	(359)	(51)
Total other comprehensive income (loss)	(94,391)	(94,332)	(94,332)		0	(94,332)	0	(59)
Comprehensive Income (Loss)	314,637	314,747	315,106				(359)	(110)
Issuance of common stock		0	0	$ 0	0	0	0	0
Issuance of common stock, shares				4,000
Repurchase of common stock		(6,572)	(6,572)	$ 0	(6,572)	0	0	0
Repurchase of common stock, shares				(6,000)
Restricted stock units expensed		3,292	3,292	$ 3,292	0	0	0	0
Restricted stock units expensed, shares				0
Adjustment of redeemable noncontrolling interests		(12,035)	(12,035)	$ 0	(12,035)	0	0	12,035
Adjustment of redeemable noncontrolling interests, shares				0
Purchase of noncontrolling interest		600	600	$ 600	0	0	0	(7,065)
Purchase of noncontrolling interest, shares				0
Other		2,708	2,741	$ 1,309	1,432	0	(33)	(2,119)
Other, shares				0
Balance at Sep. 30, 2018		9,780,154	9,783,992	$ 3,392,322	6,576,562	(184,892)	(3,838)
Balance, shares at Sep. 30, 2018				13,884,000
Balance, redeemable noncontrolling interests at Sep. 30, 2018								158,739
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
Net income (loss)	1,287,949	1,275,177	1,279,362		1,279,362	0	(4,185)	12,772
Total other comprehensive income (loss)	326,233	326,282	326,282		0	326,282	0	(49)
Comprehensive Income (Loss)	1,614,182	1,601,459	1,605,644				(4,185)	12,723
Issuance of common stock		43	43	$ 43	0	0	0	0
Issuance of common stock, shares				7,000
Repurchase of common stock	$ (81,100)	(81,998)	(81,998)	$ 0	(81,998)	0	0	0
Repurchase of common stock, shares	(77,991)			(80,000)
Restricted stock units expensed		14,282	14,282	$ 14,282	0	0	0	0
Restricted stock units expensed, shares				0
Adjustment to Nephila purchase price allocation			0	$ 0	0	0			$ (8,250)	$ (8,250)	$ 51
Adjustment to Nephila purchase price allocation, shares				0
Adjustment of redeemable noncontrolling interests		9,464	9,464	$ 0	9,464	0	0	(9,464)
Adjustment of redeemable noncontrolling interests, shares				0
Purchase of noncontrolling interest		(4,219)	(4,219)	$ (4,219)	0	0	0	(4,542)
Purchase of noncontrolling interest, shares				0
Other		3,787	(415)	$ (165)	(250)	0	4,202	(7,228)
Other, shares				0
Balance at Sep. 30, 2019	$ 10,634,870	10,634,870	10,623,454	$ 3,402,934	6,988,888	231,632	11,416
Balance, shares at Sep. 30, 2019				13,815,000
Balance, redeemable noncontrolling interests at Sep. 30, 2019	165,602							165,602
Balance at Jun. 30, 2019		10,413,081	10,396,365	$ 3,400,964	6,808,201	187,200	16,716
Balance, shares at Jun. 30, 2019				13,826,000
Balance, redeemable noncontrolling interests at Jun. 30, 2019								151,297
Net income (loss)	203,953	200,047	205,637		205,637	0	(5,590)	3,906
Total other comprehensive income (loss)	44,374	44,432	44,432		0	44,432	0	(58)
Comprehensive Income (Loss)	248,327	244,479	250,069				(5,590)	3,848
Issuance of common stock		43	43	$ 43	0	0	0	0
Issuance of common stock, shares				1,000
Repurchase of common stock		(12,732)	(12,732)	$ 0	(12,732)	0	0	0
Repurchase of common stock, shares				(12,000)
Restricted stock units expensed		2,410	2,410	$ 2,410	0	0	0	0
Restricted stock units expensed, shares				0
Adjustment of redeemable noncontrolling interests		(12,221)	(12,221)	$ 0	(12,221)	0	0	12,221
Adjustment of redeemable noncontrolling interests, shares				0
Purchase of noncontrolling interest		(483)	(483)	$ (483)	0	0	0	483
Purchase of noncontrolling interest, shares				0
Other		293	3	$ 0	3	0	290	(2,247)
Other, shares				0
Balance at Sep. 30, 2019	10,634,870	$ 10,634,870	$ 10,623,454	$ 3,402,934	$ 6,988,888	$ 231,632	$ 11,416
Balance, shares at Sep. 30, 2019				13,815,000
Balance, redeemable noncontrolling interests at Sep. 30, 2019	$ 165,602							$ 165,602