Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Preference shares	Common shares	Additional paid-in capital	Accumulated other comprehensive (loss) 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]
Issuance of shares		0
Repurchase of shares			(1,052)	(148,608)
Offering expenses				0
Exercise of options and issuance of restricted stock awards			147	(61)
Change in redeemable noncontrolling interests				(306)
Change in net unrealized gains on investments					(1,272)
Cumulative effect of adoption of ASU 2016-09 (Note 2)						2,213
Net income	$ 274,685					346,624
Net income attributable to redeemable noncontrolling interests	(71,939)					(71,939)
Dividends on common shares						(25,877)
Dividends on preference shares	(11,191)					(11,191)
Ending balance at Jun. 30, 2017	4,955,255	400,000	40,282	67,583	(139)	4,447,529
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]
Issuance of shares		250,000
Repurchase of shares			0	0
Offering expenses				(8,498)
Exercise of options and issuance of restricted stock awards			239	5,910
Change in redeemable noncontrolling interests				327
Change in net unrealized gains on investments					(1,325)
Cumulative effect of adoption of ASU 2016-09 (Note 2)						0
Net income	259,692					344,074
Net income attributable to redeemable noncontrolling interests	(84,382)					(84,382)
Dividends on common shares						(26,468)
Dividends on preference shares	(11,191)					(11,191)
Ending balance at Jun. 30, 2018	$ 4,860,061	$ 650,000	$ 40,263	$ 35,094	$ (1,101)	$ 4,135,805