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			(2,741)	(306,693)
Exercise of options and issuance of restricted stock awards			196	13,112
Change in redeemable noncontrolling interests				(1,040)
Change in net unrealized gains on investments					513
Cumulative effect of adoption of ASU 2016-09 (Note 2)						0
Net (loss) income	$ 538,798					538,798
Net loss (income) attributable to redeemable noncontrolling interests	(110,867)					(110,867)
Dividends on common shares						(38,886)
Dividends on preference shares	(16,786)					(16,786)
End of period at Sep. 30, 2016	4,807,790	400,000	41,156	213,053	2,621	4,150,960
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			(1,322)	(187,269)
Exercise of options and issuance of restricted stock awards			164	3,870
Change in redeemable noncontrolling interests				(307)
Change in net unrealized gains on investments					(972)
Cumulative effect of adoption of ASU 2016-09 (Note 2)						2,213
Net (loss) income	(356,870)					(356,870)
Net loss (income) attributable to redeemable noncontrolling interests	132,338					132,338
Dividends on common shares						(38,624)
Dividends on preference shares	(16,786)					(16,786)
End of period at Sep. 30, 2017	$ 4,403,012	$ 400,000	$ 40,029	$ 32,852	$ 161	$ 3,929,970