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 balance at Dec. 31, 2016		$ 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)
Cumulative effect of adoption of ASU 2016-09 (Note 2)						2,213
Net income	$ 97,947					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)
Ending balance at Mar. 31, 2017	4,860,837	400,000	40,785	131,173	(358)	4,289,237
Beginning balance at Dec. 31, 2017	4,391,375	400,000	40,024	37,355	224	3,913,772
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Repurchase of shares			0	0
Exercise of options and issuance of restricted stock awards			222	1,141
Change in redeemable noncontrolling interests				56
Change in net unrealized gains on investments					(30)
Cumulative effect of adoption of ASU 2016-09 (Note 2)						0
Net income	62,308					92,207
Net income attributable to redeemable noncontrolling interests	(29,899)					(29,899)
Dividends on common shares						(13,224)
Dividends on preference shares	(5,595)					(5,595)
Ending balance at Mar. 31, 2018	$ 4,436,253	$ 400,000	$ 40,246	$ 38,552	$ 194	$ 3,957,261