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				(20,058)					21,791
Share-based compensation expense				15,067
Unrealized gains arising during the period, net of reclassification adjustment	$ 328,036					328,036
Foreign currency translation adjustment	5,219						5,219
Net income (loss)	285,828							285,828
Preferred share dividends								(21,313)
Common share dividends								(68,750)
Shares repurchased	(9,414)								(9,414)
Balance at end of period at Jun. 30, 2019	5,566,477	775,000	2,206	2,303,592	156,145	159,671	(3,526)	6,108,577	(3,779,043)
Balance at beginning of period at Mar. 31, 2019		775,000	2,206	2,296,639	29,096	35,178	(6,082)	5,976,603	(3,779,388)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares repurchased		0
Shares reissued				(756)					756
Share-based compensation expense				7,709
Unrealized gains arising during the period, net of reclassification adjustment	124,493					124,493
Foreign currency translation adjustment	2,556						2,556
Net income (loss)	177,043							177,043
Preferred share dividends								(10,656)
Common share dividends								(34,413)
Shares repurchased	(411)								(411)
Balance at end of period at Jun. 30, 2019	5,566,477	775,000	2,206	2,303,592	156,145	159,671	(3,526)	6,108,577	(3,779,043)
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				(19,151)					21,040
Share-based compensation expense				19,293
Unrealized gains arising during the period, net of reclassification adjustment	113,780					113,780
Foreign currency translation adjustment	(3,891)						(3,891)
Net income (loss)	(57,783)							(57,783)
Preferred share dividends								(15,125)
Common share dividends								(70,749)
Shares repurchased	(8,602)								(8,602)
Balance at end of period at Jun. 30, 2020	5,297,820	550,000	2,206	2,317,354	281,599	295,301	(13,702)	5,913,029	(3,766,368)
Balance at beginning of period at Mar. 31, 2020		550,000	2,206	2,307,998	(89,919)	(72,383)	(17,536)	5,836,007	(3,766,714)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares repurchased		0
Shares reissued				(456)					456
Share-based compensation expense				9,812
Unrealized gains arising during the period, net of reclassification adjustment	367,684					367,684
Foreign currency translation adjustment	3,834						3,834
Net income (loss)	120,040							120,040
Preferred share dividends								(7,563)
Common share dividends								(35,455)
Shares repurchased	(110)								(110)
Balance at end of period at Jun. 30, 2020	$ 5,297,820	$ 550,000	$ 2,206	$ 2,317,354	$ 281,599	$ 295,301	$ (13,702)	$ 5,913,029	$ (3,766,368)