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
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
Balance at beginning of period at Dec. 31, 2017		$ 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]
Unrealized gains (losses) arising during the period, net of reclassification adjustment	$ (180,332)					(180,332)
Foreign currency translation adjustment	(6,864)						(6,864)
Net income	230,812							230,812
Preferred share dividends								(31,969)
Common share dividends								(100,525)
Shares repurchased	(8,699)								(8,699)
Shares reissued				(21,935)					23,644
Share-based compensation expense				26,876
Balance at end of period at Sep. 30, 2018	5,272,272	775,000	2,206	2,304,107	(162,312)	(157,868)	(4,444)	6,145,482	(3,792,211)
Balance at beginning of period at Jun. 30, 2018		775,000	2,206	2,295,633	(163,168)	(157,730)	(5,438)	6,135,625	(3,792,291)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Unrealized gains (losses) arising during the period, net of reclassification adjustment	(137)					(138)
Foreign currency translation adjustment	994						994
Net income	54,095							54,095
Preferred share dividends								(10,656)
Common share dividends								(33,582)
Shares repurchased	0								(23)
Shares reissued				(56)					103
Share-based compensation expense				8,530
Balance at end of period at Sep. 30, 2018	5,272,272	775,000	2,206	2,304,107	(162,312)	(157,868)	(4,444)	6,145,482	(3,792,211)
Balance at beginning of period 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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Unrealized gains (losses) arising during the period, net of reclassification adjustment	352,797					352,797
Foreign currency translation adjustment	609						609
Net income	324,227							324,227
Preferred share dividends								(31,969)
Common share dividends								(103,168)
Shares repurchased	(9,445)								(9,445)
Shares reissued				(20,114)					21,848
Share-based compensation expense				21,014
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]
Unrealized gains (losses) arising during the period, net of reclassification adjustment	24,761					24,761
Foreign currency translation adjustment	(4,610)						(4,610)
Net income	38,401							38,401
Preferred share dividends								(10,656)
Common share dividends								(34,420)
Shares repurchased	(31)								(31)
Shares reissued				(57)					57
Share-based compensation expense				5,948
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)