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Consolidated Statements Of Changes In Shareholders' Equity (USD $) In Thousands, unless otherwise specified	Total	Preference shares	Common stock	Additional paid-in capital	Accumulated other comprehensive income	Retained earnings	Noncontrolling interest
Beginning of period at Dec. 31, 2011		$ 550,000	$ 51,543	$ 0	$ 11,760	$ 2,991,890
Statement of Changes in Stockholders' Equity
Issuance of shares		0
Repurchase of shares		(150,000)	(6,399)	(27,376)		(460,647)
Offering expenses				0
Change in redeemable noncontrolling interest				9,091
Exercise of options and issuance of restricted stock awards (Notes 12 and 17)			398	18,285
Change in net unrealized gains on investments	(1,914)				1,914
Portion of other-than-temporary impairments recognized in other comprehensive income (loss)					(52)
Net income	748,949us-gaap_ProfitLoss					748,949
Net income attributable to noncontrolling interests (Note 10)	(148,040)					(148,040)
Dividends on common shares						(53,356)
Dividends on preference shares	(34,895)us-gaap_DividendsPreferredStock					(34,895)
Noncontrolling interest (Note 10)							3,991
Total shareholders’ equity	3,507,056
End of period at Dec. 31, 2012		400,000	45,542	0	13,622	3,043,901
Statement of Changes in Stockholders' Equity
Issuance of shares		275,000
Repurchase of shares		(275,000)	(2,451)	(1,702)		(203,703)
Offering expenses				(9,144)
Change in redeemable noncontrolling interest				318
Exercise of options and issuance of restricted stock awards (Notes 12 and 17)			555	10,528
Change in net unrealized gains on investments	9,491				(9,491)
Portion of other-than-temporary impairments recognized in other comprehensive income (loss)					0
Net income	841,768us-gaap_ProfitLoss					841,768
Net income attributable to noncontrolling interests (Note 10)	(151,144)					(151,144)
Dividends on common shares						(49,267)
Dividends on preference shares	(24,948)us-gaap_DividendsPreferredStock					(24,948)
Noncontrolling interest (Note 10)							0
Total shareholders’ equity	3,904,384
End of period at Dec. 31, 2013		400,000	43,646	0	4,131	3,456,607
Statement of Changes in Stockholders' Equity
Issuance of shares		0
Repurchase of shares		0	(5,355)	(11,702)		(497,175)
Offering expenses				0
Change in redeemable noncontrolling interest				1,274
Exercise of options and issuance of restricted stock awards (Notes 12 and 17)			151	10,428
Change in net unrealized gains on investments	715				(715)
Portion of other-than-temporary impairments recognized in other comprehensive income (loss)					0
Net income	686,256us-gaap_ProfitLoss					686,256
Net income attributable to noncontrolling interests (Note 10)	(153,538)					(153,538)
Dividends on common shares						(45,912)
Dividends on preference shares	(22,381)us-gaap_DividendsPreferredStock					(22,381)
Noncontrolling interest (Note 10)							0
Total shareholders’ equity	3,865,715
End of period at Dec. 31, 2014		$ 400,000	$ 38,442	$ 0	$ 3,416	$ 3,423,857