CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - 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]	Noncontrolling interest [Member]
Total shareholders' equity at Dec. 31, 2013			$ 627,843	$ 2,174	$ 2,240,125	$ 117,825	$ 124,945	$ (7,120)	$ 5,062,706	$ (2,232,711)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued			0	17	158
Shares repurchased			0						0
Cost of treasury shares reissued					(12,054)					12,054
Issue costs on newly issued preferred shares					0
Stock options exercised					5,405
Share-based compensation expense					51,382
Unrealized gains (losses) arising during the year, net of reclassification adjustment	$ (153,137)						(153,137)
Non-credit portion of OTTI losses							0
Foreign currency translation adjustment	(10,262)							(10,262)
Net income	804,564								804,564
Amounts attributable from noncontrolling interests	6,181								6,181
Preferred share dividends									(40,088)
Common share dividends									(117,859)
Shares repurchased for treasury					0					(543,202)
Total shareholders' equity at Dec. 31, 2014	$ 5,879,940	$ 5,821,121	627,843	2,191	2,285,016	(45,574)	(28,192)	(17,382)	5,715,504	(2,763,859)	$ 58,819
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends declared per common share (in usd per share)	$ 1.10
Shares issued			0	11	3,416
Shares repurchased			0						0
Cost of treasury shares reissued					(17,958)					17,958
Issue costs on newly issued preferred shares					0
Stock options exercised					559
Share-based compensation expense					30,355
Unrealized gains (losses) arising during the year, net of reclassification adjustment	$ (121,393)						(121,393)
Non-credit portion of OTTI losses							0
Foreign currency translation adjustment	(21,498)							(21,498)
Net income	641,631								641,631
Amounts attributable from noncontrolling interests	0								0
Preferred share dividends									(40,069)
Common share dividends									(122,713)
Shares repurchased for treasury					(60,000)					(264,538)
Total shareholders' equity at Dec. 31, 2015	$ 5,866,882	5,866,882	627,843	2,202	2,241,388	(188,465)	(149,585)	(38,880)	6,194,353	(3,010,439)	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends declared per common share (in usd per share)	$ 1.22
Shares issued			550,000	4	220
Shares repurchased			(51,769)						(1,309)
Cost of treasury shares reissued					(19,303)					20,691
Issue costs on newly issued preferred shares					(18,055)
Stock options exercised					0
Share-based compensation expense					35,607
Unrealized gains (losses) arising during the year, net of reclassification adjustment	$ 67,262						67,262
Non-credit portion of OTTI losses							0
Foreign currency translation adjustment	(638)							(638)
Net income	513,368								513,368
Amounts attributable from noncontrolling interests	0								0
Preferred share dividends									(46,597)
Common share dividends									(132,188)
Shares repurchased for treasury					60,000					(571,805)
Total shareholders' equity at Dec. 31, 2016	$ 6,272,370	$ 6,272,370	$ 1,126,074	$ 2,206	$ 2,299,857	$ (121,841)	$ (82,323)	$ (39,518)	$ 6,527,627	$ (3,561,553)	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends declared per common share (in usd per share)	$ 1.43