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 (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
Balance at beginning of period at Dec. 31, 2021			$ 550,000	$ 2,206	$ 2,346,179	$ 56,536	$ 62,155	$ (5,619)	$ 6,204,745	$ (3,749,010)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Treasury shares reissued					(30,150)					31,695
Share-based compensation expense					25,478
Unrealized gains (losses) arising during the period, net of reclassification adjustment		$ (777,232)					(777,232)
Foreign currency translation adjustment		(3,418)						(3,418)
Net income		183,982							183,982
Preferred share dividends	[1]								(15,125)
Common share dividends	[1]								(74,922)
Shares repurchased										(48,333)
Balance at end of period at Jun. 30, 2022		4,702,631	550,000	2,206	2,341,507	(724,114)	(715,077)	(9,037)	6,298,680	(3,765,648)
Balance at beginning of period at Mar. 31, 2022			550,000	2,206	2,328,986	(338,300)	(335,456)	(2,844)	6,308,712	(3,731,064)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Treasury shares reissued					(772)					772
Share-based compensation expense					13,293
Unrealized gains (losses) arising during the period, net of reclassification adjustment		(379,621)					(379,621)
Foreign currency translation adjustment		(6,193)						(6,193)
Net income		34,778							34,778
Preferred share dividends	[1]								(7,563)
Common share dividends	[1]								(37,247)
Shares repurchased										(35,356)
Balance at end of period at Jun. 30, 2022		4,702,631	550,000	2,206	2,341,507	(724,114)	(715,077)	(9,037)	6,298,680	(3,765,648)
Balance at beginning of period at Dec. 31, 2022		4,639,910	550,000	2,206	2,366,253	(760,300)	(743,695)	(16,605)	6,247,022	(3,765,271)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Treasury shares reissued					(32,796)					34,500
Share-based compensation expense					27,728
Unrealized gains (losses) arising during the period, net of reclassification adjustment		127,143					127,143
Foreign currency translation adjustment		2,648						2,648
Net income		330,773							330,773
Preferred share dividends	[1]								(15,125)
Common share dividends	[1]								(76,769)
Shares repurchased										(17,051)
Balance at end of period at Jun. 30, 2023		5,020,961	550,000	2,206	2,361,185	(630,509)	(616,552)	(13,957)	6,485,901	(3,747,822)
Balance at beginning of period at Mar. 31, 2023			550,000	2,206	2,347,637	(571,896)	(554,756)	(17,140)	6,381,201	(3,748,919)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Treasury shares reissued					(2,203)					2,203
Share-based compensation expense					15,751
Unrealized gains (losses) arising during the period, net of reclassification adjustment		(61,796)					(61,796)
Foreign currency translation adjustment		3,183						3,183
Net income		150,674							150,674
Preferred share dividends	[1]								(7,563)
Common share dividends	[1]								(38,411)
Shares repurchased										(1,106)
Balance at end of period at Jun. 30, 2023		$ 5,020,961	$ 550,000	$ 2,206	$ 2,361,185	$ (630,509)	$ (616,552)	$ (13,957)	$ 6,485,901	$ (3,747,822)

[1] Refer to Note 9 'Shareholders' Equity' for details on dividends declared and paid related to the Company's common and preferred shares.