CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Thousands	Total	Parent [Member]	Preferred shares [Member]	Common shares (par value) [Member]	Additional paid-in capital [Member]	Accumulated other comprehensive income [Member]	Unrealized gains (losses) on available for sale investments, net of tax [Member]	Cumulative foreign currency translation adjustments, net of tax [Member]	Retained earnings [Member]	Treasury shares, at cost [Member]
Total shareholders’ equity at Dec. 31, 2015			$ 627,843	$ 2,202	$ 2,241,388	$ (188,465)	$ (149,585)	$ (38,880)	$ 6,194,353	$ (3,010,439)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares repurchased			(2,843)
Shares issued				4	(4)
Cost of treasury shares reissued					(19,647)					21,033
Shares repurchased for treasury					60,000					(449,086)
Share-based compensation expense					26,129
Unrealized gains arising during the period, net of reclassification adjustment	$ 281,276						281,276
Foreign currency translation adjustment	5,694							5,694
Net income (loss)	364,460								364,460
Preferred share dividends									(29,906)
Common share dividends									(98,334)
Total shareholders’ equity at Sep. 30, 2016		$ 6,025,658	625,000	2,206	2,307,866	98,505	131,691	(33,186)	6,430,573	(3,438,492)
Total shareholders’ equity at Dec. 31, 2016	6,272,370		1,126,074	2,206	2,299,857	(121,841)	(82,323)	(39,518)	6,527,627	(3,561,553)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares repurchased			(351,074)
Shares issued				0	0
Cost of treasury shares reissued					(39,033)					39,917
Shares repurchased for treasury					0					(285,659)
Share-based compensation expense					30,692
Unrealized gains arising during the period, net of reclassification adjustment	216,630						216,630
Foreign currency translation adjustment	46,824							46,824
Net income (loss)	(341,541)								(341,541)
Preferred share dividends									(36,154)
Common share dividends									(98,273)
Total shareholders’ equity at Sep. 30, 2017	$ 5,454,699	$ 5,454,699	$ 775,000	$ 2,206	$ 2,291,516	$ 141,613	$ 134,307	$ 7,306	$ 6,051,659	$ (3,807,295)