CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Thousands	Total	Preferred shares	Common shares (par value)	Additional paid-in capital	Accumulated other comprehensive income	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
Balance at beginning of period at Dec. 31, 2018		$ 775,000	$ 2,206	$ 2,308,583	$ (177,110)	$ (168,365)	$ (8,745)	$ 5,912,812	$ (3,791,420)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares repurchased		0
Shares reissued				(20,114)					21,848
Share-based compensation expense				21,014
Unrealized gains arising during the period, net of reclassification adjustment	$ 352,797					352,797
Foreign currency translation adjustment	609						609
Net income (loss)	324,227							324,227
Preferred share dividends								(31,969)
Common share dividends								(103,168)
Shares repurchased	(9,445)								(9,445)
Balance at end of period at Sep. 30, 2019	5,585,870	775,000	2,206	2,309,483	176,296	184,432	(8,136)	6,101,902	(3,779,017)
Balance at beginning of period at Jun. 30, 2019		775,000	2,206	2,303,592	156,145	159,671	(3,526)	6,108,577	(3,779,043)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares repurchased		0
Shares reissued				(57)					57
Share-based compensation expense				5,948
Unrealized gains arising during the period, net of reclassification adjustment	24,761					24,761
Foreign currency translation adjustment	(4,610)						(4,610)
Net income (loss)	38,401							38,401
Preferred share dividends								(10,656)
Common share dividends								(34,420)
Shares repurchased	(31)								(31)
Balance at end of period at Sep. 30, 2019	5,585,870	775,000	2,206	2,309,483	176,296	184,432	(8,136)	6,101,902	(3,779,017)
Balance at beginning of period at Dec. 31, 2019	5,544,008	775,000	2,206	2,317,212	171,710	181,521	(9,811)	6,056,686	(3,778,806)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares repurchased		(225,000)
Shares reissued				(19,348)					21,236
Share-based compensation expense				27,332
Unrealized gains arising during the period, net of reclassification adjustment	179,982					179,982
Foreign currency translation adjustment	(1,581)						(1,581)
Net income (loss)	(123,167)							(123,167)
Preferred share dividends								(22,688)
Common share dividends								(106,194)
Shares repurchased	(8,685)								(8,685)
Balance at end of period at Sep. 30, 2020	5,265,895	550,000	2,206	2,325,196	350,111	361,503	(11,392)	5,804,637	(3,766,255)
Balance at beginning of period at Jun. 30, 2020		550,000	2,206	2,317,354	281,599	295,301	(13,702)	5,913,029	(3,766,368)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares repurchased		0
Shares reissued				(197)					197
Share-based compensation expense				8,039
Unrealized gains arising during the period, net of reclassification adjustment	66,202					66,202
Foreign currency translation adjustment	2,310						2,310
Net income (loss)	(65,382)							(65,382)
Preferred share dividends								(7,563)
Common share dividends								(35,447)
Shares repurchased	(84)								(84)
Balance at end of period at Sep. 30, 2020	$ 5,265,895	$ 550,000	$ 2,206	$ 2,325,196	$ 350,111	$ 361,503	$ (11,392)	$ 5,804,637	$ (3,766,255)