Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Preference shares	Common shares	Additional paid-in capital	Accumulated other comprehensive loss	Retained earnings
Beginning balance at Dec. 31, 2018		$ 650,000	$ 42,207	$ 296,099	$ (1,433)	$ 4,058,207
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares			1,739	248,259
Repurchase of shares		0	0	0
Exercise of options and issuance of restricted stock awards			213	(466)
Change in redeemable noncontrolling interests				(3)
Change in net unrealized losses on investments, net of tax	$ (37)				(37)
Foreign currency translation adjustments, net of tax	0				0
Net income	353,128					353,128
Net income attributable to redeemable noncontrolling interests	(70,222)					(70,222)
Dividends on common shares	(14,500)					(14,469)
Dividends on preference shares	(9,189)					(9,189)
Ending balance at Mar. 31, 2019	5,554,033	650,000	44,159	543,889	(1,470)	4,317,455
Beginning balance at Dec. 31, 2019	5,971,367	650,000	44,148	568,277	(1,939)	4,710,881
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares			0	0
Repurchase of shares		(125,000)	(406)	(62,215)
Exercise of options and issuance of restricted stock awards			292	(3,105)
Change in redeemable noncontrolling interests				(349)
Change in net unrealized losses on investments, net of tax	(657)				(657)
Foreign currency translation adjustments, net of tax	932				932
Net income	25,173					25,173
Net income attributable to redeemable noncontrolling interests	(98,091)					(98,091)
Dividends on common shares	(15,400)					(15,359)
Dividends on preference shares	(9,056)
Ending balance at Mar. 31, 2020	$ 5,683,526	$ 525,000	$ 44,034	$ 502,608	$ (1,664)	$ 4,613,548