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 (loss) [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					(17,631)					19,017
Shares repurchased for treasury					60,000					(323,050)
Share-based compensation expense					18,804
Unrealized gains arising during the period, net of reclassification adjustment	$ 247,582						247,582
Foreign currency translation adjustment	3,972							3,972
Net income	177,846								177,846
Preferred share dividends									(19,938)
Common share dividends									(66,458)
Total shareholders’ equity at Jun. 30, 2016		$ 5,964,183	625,000	2,206	2,302,557	63,089	97,997	(34,908)	6,285,803	(3,314,472)
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					(38,840)					39,725
Shares repurchased for treasury					0					(282,313)
Share-based compensation expense					22,506
Unrealized gains arising during the period, net of reclassification adjustment	167,411						167,411
Foreign currency translation adjustment	38,736							38,736
Net income	115,542								115,542
Preferred share dividends									(25,497)
Common share dividends									(65,871)
Total shareholders’ equity at Jun. 30, 2017	$ 5,892,695	$ 5,892,695	$ 775,000	$ 2,206	$ 2,283,523	$ 84,306	$ 85,088	$ (782)	$ 6,551,801	$ (3,804,141)