CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Preferred shares	Common stocks	Additional paid-in capital	Accumulated other comprehensive income (loss)	Unrealized gains (losses) on available for sale investments, net of tax	Cumulative foreign currency translation adjustments, net of tax	Retained earnings	Treasury shares, at cost
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adoption of ASU | Accounting Standards Update 2018-02						$ 0		$ 0
Cumulative effect of adoption of ASU | Accounting Standards Update 2016-01						0		0
Balance at beginning of year 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 issued		550,000	4	220
Shares reissued				(19,303)					20,691
Shares repurchased				60,000					(571,805)
Costs associated with issuance of preferred shares				(18,055)
Share-based compensation expense				35,607
Unrealized gains (losses) arising during the year, net of reclassification adjustment	$ 67,262					67,262
Foreign currency translation adjustment	(638)						(638)
Net income (loss)	513,368							513,368
Preferred share dividends								(46,597)
Loss on repurchase of preferred shares		(51,769)						(1,309)
Common share dividends								(132,188)
Balance at end of year 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]
Cumulative effect of adoption of ASU | Accounting Standards Update 2018-02						0		0
Cumulative effect of adoption of ASU | Accounting Standards Update 2016-01						0		0
Shares issued		0	0	0
Shares reissued				(39,368)					40,255
Shares repurchased				0					(285,858)
Costs associated with issuance of preferred shares				0
Share-based compensation expense				38,677
Unrealized gains (losses) arising during the year, net of reclassification adjustment	172,285					172,285
Foreign currency translation adjustment	41,938						41,938
Net income (loss)	(368,969)							(368,969)
Preferred share dividends								(46,810)
Loss on repurchase of preferred shares		(351,074)						0
Common share dividends								(132,182)
Balance at end of year at Dec. 31, 2017	5,341,264	775,000	2,206	2,299,166	92,382	89,962	2,420	5,979,666	(3,807,156)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adoption of ASU | Accounting Standards Update 2018-02						2,106		(2,106)
Cumulative effect of adoption of ASU | Accounting Standards Update 2016-01						(69,604)		69,604
Shares issued		0	0	0
Shares reissued				(24,088)					25,816
Shares repurchased				0					(10,080)
Costs associated with issuance of preferred shares				0
Share-based compensation expense				33,505
Unrealized gains (losses) arising during the year, net of reclassification adjustment	(190,829)					(190,829)
Foreign currency translation adjustment	(11,165)						(11,165)
Net income (loss)	43,021							43,021
Preferred share dividends								(42,625)
Loss on repurchase of preferred shares		0						0
Common share dividends								(134,748)
Balance at end of year at Dec. 31, 2018	$ 5,030,071	$ 775,000	$ 2,206	$ 2,308,583	$ (177,110)	$ (168,365)	$ (8,745)	$ 5,912,812	$ (3,791,420)