Fair Value (Details) (USD $) In Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	$ 14,406,945		$ 14,406,945		$ 12,824,389
Short-term investments, trading securities	215,692		215,692		49,397
Equities, trading securities	994,913		994,913		1,071,676
Other invested assets	40,777		40,777		111,374
Funds held - directly managed	1,158,814		1,158,814		1,542,931
Investments Fair Value Disclosure	16,817,141		16,817,141		15,599,767
Fair Value, Option, Quantitative Disclosures [Line Items]
Change in unrealized gain/loss	107,971	58,824	(28,673)	192,171
Quoted prices in active markets for identical assets (Level 1)
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	0		0		0
Short-term investments, trading securities	0		0		0
Equities, trading securities	775,619		775,619		1,028,263
Other invested assets	(13,417)		(13,417)		23,262
Funds held - directly managed	0		0		0
Investments Fair Value Disclosure	762,202		762,202		1,051,525
Quoted prices in active markets for identical assets (Level 1) | US Government and agencies [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	0		0		0
Funds held - directly managed	0		0		0
Quoted prices in active markets for identical assets (Level 1) | Non US sovereign government supranational and government related [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	0		0		0
Funds held - directly managed	0		0		0
Quoted prices in active markets for identical assets (Level 1) | Corporate [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	0		0		0
Funds held - directly managed	0		0		0
Quoted prices in active markets for identical assets (Level 1) | Asset-backed securities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	0		0		0
Quoted prices in active markets for identical assets (Level 1) | Mortgage/asset-backed securities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Funds held - directly managed					0
Quoted prices in active markets for identical assets (Level 1) | Residential mortgage-backed securities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	0		0		0
Quoted prices in active markets for identical assets (Level 1) | Other mortgage-backed securities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	0		0		0
Quoted prices in active markets for identical assets (Level 1) | Consumer noncyclical [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	138,638		138,638		186,016
Quoted prices in active markets for identical assets (Level 1) | Energy [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	96,782		96,782		118,372
Quoted prices in active markets for identical assets (Level 1) | Finance [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	89,054		89,054		112,309
Quoted prices in active markets for identical assets (Level 1) | Technology [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	78,939		78,939		119,214
Quoted prices in active markets for identical assets (Level 1) | Communications [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	76,188		76,188		110,982
Quoted prices in active markets for identical assets (Level 1) | Industrials [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	71,405		71,405		100,572
Quoted prices in active markets for identical assets (Level 1) | Consumer cyclical [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	57,087		57,087		81,595
Quoted prices in active markets for identical assets (Level 1) | Insurance [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	38,447		38,447		48,611
Quoted prices in active markets for identical assets (Level 1) | Other equities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	70,343		70,343		90,220
Quoted prices in active markets for identical assets (Level 1) | Mutual funds and exchange traded funds [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	58,736		58,736		60,372
Quoted prices in active markets for identical assets (Level 1) | Other invested assets other assets [Member ]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
Quoted prices in active markets for identical assets (Level 1) | Short-term investments [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Funds held - directly managed	0		0		0
Quoted prices in active markets for identical assets (Level 1) | Other invested assets funds held directly managed [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Funds held - directly managed	0		0		0
Quoted prices in active markets for identical assets (Level 1) | Derivative assets [Member] | Foreign exchange forward contracts [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
Quoted prices in active markets for identical assets (Level 1) | Derivative assets [Member] | Foreign currency option contracts [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
Quoted prices in active markets for identical assets (Level 1) | Derivative assets [Member] | Futures contracts [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	6,179		6,179		30,593
Quoted prices in active markets for identical assets (Level 1) | Derivative assets [Member] | Credit Default Swap, Buying Protection [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
Quoted prices in active markets for identical assets (Level 1) | Derivative assets [Member] | Credit default swaps (assumed risks) [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
Quoted prices in active markets for identical assets (Level 1) | Derivative assets [Member] | Insurance-linked securities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	23		23		1,320
Quoted prices in active markets for identical assets (Level 1) | Derivative assets [Member] | Total return swaps [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
Quoted prices in active markets for identical assets (Level 1) | Derivative assets [Member] | Interest rate swaps [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
Quoted prices in active markets for identical assets (Level 1) | Derivative liabilities [Member] | Foreign exchange forward contracts [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
Quoted prices in active markets for identical assets (Level 1) | Derivative liabilities [Member] | Foreign currency option contracts [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0
Quoted prices in active markets for identical assets (Level 1) | Derivative liabilities [Member] | Futures contracts [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	(19,126)		(19,126)		(7,956)
Quoted prices in active markets for identical assets (Level 1) | Derivative liabilities [Member] | Credit Default Swap, Buying Protection [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
Quoted prices in active markets for identical assets (Level 1) | Derivative liabilities [Member] | Credit default swaps (assumed risks) [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets					0
Quoted prices in active markets for identical assets (Level 1) | Derivative liabilities [Member] | Insurance-linked securities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	(493)		(493)		(695)
Quoted prices in active markets for identical assets (Level 1) | Derivative liabilities [Member] | Total return swaps [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
Quoted prices in active markets for identical assets (Level 1) | Derivative liabilities [Member] | Interest rate swaps [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
Quoted prices in active markets for identical assets (Level 1) | Other invested assets other liabilties [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
Significant other observable inputs (Level 2)
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	13,941,891		13,941,891		12,478,854
Short-term investments, trading securities	212,769		212,769		49,397
Equities, trading securities	212,589		212,589		0
Other invested assets	1,711		1,711		9,788
Funds held - directly managed	1,136,739		1,136,739		1,509,917
Investments Fair Value Disclosure	15,505,699		15,505,699		14,047,956
Significant other observable inputs (Level 2) | US Government and agencies [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	1,486,790		1,486,790		917,600
Funds held - directly managed	230,481		230,481		288,164
Significant other observable inputs (Level 2) | Non US sovereign government supranational and government related [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	3,258,355		3,258,355		2,819,193
Funds held - directly managed	313,284		313,284		384,553
Significant other observable inputs (Level 2) | Corporate [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	5,713,985		5,713,985		6,066,865
Funds held - directly managed	550,644		550,644		798,587
Significant other observable inputs (Level 2) | Asset-backed securities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	394,046		394,046		343,518
Significant other observable inputs (Level 2) | Mortgage/asset-backed securities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Funds held - directly managed					0
Significant other observable inputs (Level 2) | Residential mortgage-backed securities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	2,999,005		2,999,005		2,305,525
Significant other observable inputs (Level 2) | Other mortgage-backed securities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	89,710		89,710		26,153
Significant other observable inputs (Level 2) | Consumer noncyclical [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	178		178		0
Significant other observable inputs (Level 2) | Energy [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	1,051		1,051		0
Significant other observable inputs (Level 2) | Finance [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	617		617		0
Significant other observable inputs (Level 2) | Technology [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	0		0		0
Significant other observable inputs (Level 2) | Communications [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	33		33		0
Significant other observable inputs (Level 2) | Industrials [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	0		0		0
Significant other observable inputs (Level 2) | Consumer cyclical [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	86		86		0
Significant other observable inputs (Level 2) | Insurance [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	0		0		0
Significant other observable inputs (Level 2) | Other equities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	408		408		0
Significant other observable inputs (Level 2) | Mutual funds and exchange traded funds [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	210,216		210,216		0
Significant other observable inputs (Level 2) | Other invested assets other assets [Member ]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
Significant other observable inputs (Level 2) | Short-term investments [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Funds held - directly managed	42,330		42,330		38,613
Significant other observable inputs (Level 2) | Other invested assets funds held directly managed [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Funds held - directly managed	0		0		0
Significant other observable inputs (Level 2) | Derivative assets [Member] | Foreign exchange forward contracts [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	11,634		11,634		27,880
Significant other observable inputs (Level 2) | Derivative assets [Member] | Foreign currency option contracts [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	2,921		2,921		3,516
Significant other observable inputs (Level 2) | Derivative assets [Member] | Futures contracts [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	219		219		0
Significant other observable inputs (Level 2) | Derivative assets [Member] | Credit Default Swap, Buying Protection [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	4		4		93
Significant other observable inputs (Level 2) | Derivative assets [Member] | Credit default swaps (assumed risks) [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	906		906		533
Significant other observable inputs (Level 2) | Derivative assets [Member] | Insurance-linked securities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
Significant other observable inputs (Level 2) | Derivative assets [Member] | Total return swaps [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	379		379		449
Significant other observable inputs (Level 2) | Derivative assets [Member] | Interest rate swaps [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	52		52		246
Significant other observable inputs (Level 2) | Derivative liabilities [Member] | Foreign exchange forward contracts [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	(4,512)		(4,512)		(13,647)
Significant other observable inputs (Level 2) | Derivative liabilities [Member] | Foreign currency option contracts [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	(470)		(470)
Significant other observable inputs (Level 2) | Derivative liabilities [Member] | Futures contracts [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	(487)		(487)		0
Significant other observable inputs (Level 2) | Derivative liabilities [Member] | Credit Default Swap, Buying Protection [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	(2,162)		(2,162)		(2,407)
Significant other observable inputs (Level 2) | Derivative liabilities [Member] | Credit default swaps (assumed risks) [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets					(401)
Significant other observable inputs (Level 2) | Derivative liabilities [Member] | Insurance-linked securities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
Significant other observable inputs (Level 2) | Derivative liabilities [Member] | Total return swaps [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
Significant other observable inputs (Level 2) | Derivative liabilities [Member] | Interest rate swaps [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	(5,203)		(5,203)		(6,033)
Significant other observable inputs (Level 2) | Other invested assets other liabilties [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	(1,570)		(1,570)		(441)
Significant unobservable inputs (Level 3)
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	465,054		465,054		345,535
Short-term investments, trading securities	2,923		2,923		0
Equities, trading securities	6,705		6,705		43,413
Other invested assets	52,483		52,483		78,324
Funds held - directly managed	22,075		22,075		33,014
Investments Fair Value Disclosure	549,240		549,240		500,286
Significant unobservable inputs (Level 3) | US Government and agencies [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	86,782		86,782		55,124
Funds held - directly managed	355		355		368
Significant unobservable inputs (Level 3) | Non US sovereign government supranational and government related [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	0		0		0
Funds held - directly managed	0		0		0
Significant unobservable inputs (Level 3) | Corporate [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	116,428		116,428		76,982
Funds held - directly managed	0		0		0
Significant unobservable inputs (Level 3) | Asset-backed securities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	261,843		261,843		213,139
Significant unobservable inputs (Level 3) | Mortgage/asset-backed securities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Funds held - directly managed					12,118
Significant unobservable inputs (Level 3) | Residential mortgage-backed securities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	0		0		0
Significant unobservable inputs (Level 3) | Other mortgage-backed securities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	1		1		290
Significant unobservable inputs (Level 3) | Consumer noncyclical [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	0		0		0
Significant unobservable inputs (Level 3) | Energy [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	0		0		0
Significant unobservable inputs (Level 3) | Finance [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	163		163		2,486
Significant unobservable inputs (Level 3) | Technology [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	0		0		0
Significant unobservable inputs (Level 3) | Communications [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	0		0		0
Significant unobservable inputs (Level 3) | Industrials [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	0		0		0
Significant unobservable inputs (Level 3) | Consumer cyclical [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	0		0		0
Significant unobservable inputs (Level 3) | Insurance [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	0		0		0
Significant unobservable inputs (Level 3) | Other equities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	0		0		0
Significant unobservable inputs (Level 3) | Mutual funds and exchange traded funds [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	6,542		6,542		40,927
Significant unobservable inputs (Level 3) | Other invested assets other assets [Member ]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	74,018		74,018		86,278
Significant unobservable inputs (Level 3) | Short-term investments [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Funds held - directly managed	0		0		0
Significant unobservable inputs (Level 3) | Other invested assets funds held directly managed [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Funds held - directly managed	21,720		21,720		20,528
Significant unobservable inputs (Level 3) | Derivative assets [Member] | Foreign exchange forward contracts [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
Significant unobservable inputs (Level 3) | Derivative assets [Member] | Foreign currency option contracts [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
Significant unobservable inputs (Level 3) | Derivative assets [Member] | Futures contracts [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
Significant unobservable inputs (Level 3) | Derivative assets [Member] | Credit Default Swap, Buying Protection [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
Significant unobservable inputs (Level 3) | Derivative assets [Member] | Credit default swaps (assumed risks) [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
Significant unobservable inputs (Level 3) | Derivative assets [Member] | Insurance-linked securities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	251		251		0
Significant unobservable inputs (Level 3) | Derivative assets [Member] | Total return swaps [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	4,306		4,306		5,592
Significant unobservable inputs (Level 3) | Derivative assets [Member] | Interest rate swaps [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
Significant unobservable inputs (Level 3) | Derivative liabilities [Member] | Foreign exchange forward contracts [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
Significant unobservable inputs (Level 3) | Derivative liabilities [Member] | Foreign currency option contracts [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0
Significant unobservable inputs (Level 3) | Derivative liabilities [Member] | Futures contracts [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
Significant unobservable inputs (Level 3) | Derivative liabilities [Member] | Credit Default Swap, Buying Protection [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
Significant unobservable inputs (Level 3) | Derivative liabilities [Member] | Credit default swaps (assumed risks) [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets					0
Significant unobservable inputs (Level 3) | Derivative liabilities [Member] | Insurance-linked securities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	(14,633)		(14,633)		(698)
Significant unobservable inputs (Level 3) | Derivative liabilities [Member] | Total return swaps [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	(11,459)		(11,459)		(12,848)
Significant unobservable inputs (Level 3) | Derivative liabilities [Member] | Interest rate swaps [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
Significant unobservable inputs (Level 3) | Other invested assets other liabilties [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	0		0		0
US Government and agencies [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	1,573,572		1,573,572		972,724
Funds held - directly managed	230,836		230,836		288,532
Non US sovereign government supranational and government related [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	3,258,355		3,258,355		2,819,193
Funds held - directly managed	313,284		313,284		384,553
Corporate [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	5,830,413		5,830,413		6,143,847
Funds held - directly managed	550,644		550,644		798,587
Asset-backed securities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	655,889		655,889		556,657
Mortgage/asset-backed securities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Funds held - directly managed					12,118
Residential mortgage-backed securities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	2,999,005		2,999,005		2,305,525
Other mortgage-backed securities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Fixed maturities, trading securities	89,711		89,711		26,443
Consumer noncyclical [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	138,816		138,816		186,016
Energy [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	97,833		97,833		118,372
Finance [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	89,834		89,834		114,795
Technology [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	78,939		78,939		119,214
Communications [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	76,221		76,221		110,982
Industrials [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	71,405		71,405		100,572
Consumer cyclical [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	57,173		57,173		81,595
Insurance [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	38,447		38,447		48,611
Other equities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	70,751		70,751		90,220
Mutual funds and exchange traded funds [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Equities, trading securities	275,494		275,494		101,299
Other invested assets other assets [Member ]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	74,018		74,018		86,278
Short-term investments [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Funds held - directly managed	42,330		42,330		38,613
Short-term investments [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Change in unrealized gain/loss	194	7	(446)	(2,418)
Other invested assets funds held directly managed [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Funds held - directly managed	21,720		21,720		20,528
Derivative assets [Member] | Foreign exchange forward contracts [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	11,634		11,634		27,880
Derivative assets [Member] | Foreign currency option contracts [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	2,921		2,921		3,516
Derivative assets [Member] | Futures contracts [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	6,398		6,398		30,593
Derivative assets [Member] | Credit Default Swap, Buying Protection [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	4		4		93
Derivative assets [Member] | Credit default swaps (assumed risks) [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	906		906		533
Derivative assets [Member] | Insurance-linked securities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	274		274		1,320
Derivative assets [Member] | Total return swaps [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	4,685		4,685		6,041
Derivative assets [Member] | Interest rate swaps [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	52		52		246
Derivative liabilities [Member] | Foreign exchange forward contracts [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	(4,512)		(4,512)		(13,647)
Derivative liabilities [Member] | Foreign currency option contracts [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	(470)		(470)
Derivative liabilities [Member] | Futures contracts [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	(19,613)		(19,613)		(7,956)
Derivative liabilities [Member] | Credit Default Swap, Buying Protection [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	(2,162)		(2,162)		(2,407)
Derivative liabilities [Member] | Credit default swaps (assumed risks) [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets					(401)
Derivative liabilities [Member] | Insurance-linked securities [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	(15,126)		(15,126)		(1,393)
Derivative liabilities [Member] | Total return swaps [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	(11,459)		(11,459)		(12,848)
Derivative liabilities [Member] | Interest rate swaps [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	(5,203)		(5,203)		(6,033)
Other invested assets other liabilties [Member]
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]
Other invested assets	(1,570)		(1,570)		(441)
Fixed maturities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Change in unrealized gain/loss	130,907	165,665	(9,321)	264,762
Other invested assets [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Change in unrealized gain/loss	(3,689)	(226)	(3,333)	(142)
Equity securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Change in unrealized gain/loss	(30,197)	(126,610)	(14,079)	(101,198)
Funds Held - Directly Managed investment [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Change in unrealized gain/loss	$ 10,756	$ 19,988	$ (1,494)	$ 31,167