Consolidated Statements Of Changes In Shareholders' Equity - USD ($) $ in Thousands	Total	Preference Shares	Common Shares	Additional Paid-In Capital	Accumulated Other Comprehensive Income	Retained Earnings
Beginning of period at Dec. 31, 2015		$ 400,000	$ 43,701	$ 507,674	$ 2,108	$ 3,778,701
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Repurchase of shares			(769)	(84,397)
Exercise of options and issuance of restricted stock awards			163	(778)
Change in redeemable noncontrolling interests				(77)
Change in net unrealized gains on investments					(443)
Net income	$ 178,181					178,181
Net income attributable to redeemable noncontrolling interests	(44,591)					(44,591)
Dividends on common shares						(13,285)
Dividends on preference shares	(5,595)					(5,595)
End of period at Mar. 31, 2016	4,760,593	400,000	43,095	422,422	1,665	3,893,411
Beginning of period at Dec. 31, 2016	4,866,577	400,000	41,187	216,558	1,133	4,207,699
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Repurchase of shares			(550)	(79,459)
Exercise of options and issuance of restricted stock awards			148	(5,840)
Change in redeemable noncontrolling interests				(86)
Change in net unrealized gains on investments					(1,491)
Net income	132,274					132,274
Net income attributable to redeemable noncontrolling interests	(34,327)					(34,327)
Dividends on common shares						(13,027)
Dividends on preference shares	(5,595)					(5,595)
End of period at Mar. 31, 2017	$ 4,860,837	$ 400,000	$ 40,785	$ 131,173	$ (358)	$ 4,289,237