Unaudited Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Non-cumulative preferred shares	Common shares	Additional paid-in capital	Retained earnings	Accumulated other comprehensive income (loss), net of deferred income tax	Unrealized appreciation (decline) in value of available-for-sale securities, net of deferred income tax	Foreign currency translation adjustments	Common shares held in treasury, at cost	Convertible non-voting common equivalent preferred shares
Balance at beginning of year, as adjusted					$ 7,996,387		$ (27,641)
Balance at beginning of year at Dec. 31, 2016		$ 772,555	$ 582	$ 531,687	7,996,701	$ (114,541)	(27,641)	$ (86,900)	$ (2,034,570)	$ 1,101,304
Cumulative effect of an accounting change at Dec. 31, 2016					(314)		0
Preferred shares redeemed		0
Preferred shares converted to common shares				611,653						(611,677)
Common shares issued, net			27
Loss on redemption of preferred shares	$ 0			0	0
All other				53,544
Net income	473,134				473,134
Net (income) loss attributable to noncontrolling interests	(34,840)				(34,840)
Preferred share dividends	(22,567)				(22,567)
Unrealized holding gains (losses) arising during period, net of reclassification adjustment							171,493
Unrealized holding gains (losses) arising during period attributable to noncontrolling interests							0
Foreign currency translation adjustments	21,421							21,421
Foreign currency translation adjustments attributable to noncontrolling interests								68
Shares repurchased for treasury									(16,773)
Balance at end of period at Jun. 30, 2017	8,898,887	772,555	609	1,196,884	8,412,114	78,441	143,852	(65,411)	(2,051,343)	489,627
Non-redeemable noncontrolling interests, end of period at Jun. 30, 2017	877,456
Total shareholders’ equity at Jun. 30, 2017	9,776,343
Loss on redemption of preferred shares	0
Net income	199,099
Net (income) loss attributable to noncontrolling interests	(13,932)
Preferred share dividends	(11,349)
Foreign currency translation adjustments	18,297
Balance at end of period at Jun. 30, 2017	8,898,887	772,555	609	1,196,884	8,412,114	78,441	143,852	(65,411)	(2,051,343)	489,627
Non-redeemable noncontrolling interests, end of period at Jun. 30, 2017	877,456
Total shareholders’ equity at Jun. 30, 2017	9,776,343
Balance at beginning of year, as adjusted					8,712,683		7,606
Balance at beginning of year at Dec. 31, 2017	9,196,602	872,555	611	1,230,617	8,562,889	118,044	157,400	(39,356)	(2,077,741)	489,627
Cumulative effect of an accounting change at Dec. 31, 2017					149,794		(149,794)
Preferred shares redeemed		(92,555)
Preferred shares converted to common shares				489,608						(489,627)
Common shares issued, net			22
Loss on redemption of preferred shares	(2,710)			2,710	(2,710)
All other				37,671
Net income	422,731				422,731
Net (income) loss attributable to noncontrolling interests	(28,662)				(28,662)
Preferred share dividends	(20,840)				(20,840)
Unrealized holding gains (losses) arising during period, net of reclassification adjustment							(152,844)
Unrealized holding gains (losses) arising during period attributable to noncontrolling interests							1,885
Foreign currency translation adjustments	(11,313)							(11,313)
Foreign currency translation adjustments attributable to noncontrolling interests								(135)
Shares repurchased for treasury	(173,600)								(188,788)
Balance at end of period at Jun. 30, 2018	9,163,755	780,000	633	1,760,606	9,083,202	(194,157)	(143,353)	(50,804)	(2,266,529)	0
Non-redeemable noncontrolling interests, end of period at Jun. 30, 2018	861,153
Total shareholders’ equity at Jun. 30, 2018	10,024,908
Loss on redemption of preferred shares	0
Net income	256,347
Net (income) loss attributable to noncontrolling interests	(12,701)
Preferred share dividends	(10,403)
Foreign currency translation adjustments	(12,595)
Balance at end of period at Jun. 30, 2018	9,163,755	$ 780,000	$ 633	$ 1,760,606	$ 9,083,202	$ (194,157)	$ (143,353)	$ (50,804)	$ (2,266,529)	$ 0
Non-redeemable noncontrolling interests, end of period at Jun. 30, 2018	861,153
Total shareholders’ equity at Jun. 30, 2018	$ 10,024,908