Funds Held - Directly Managed Cost, unrealized gains and losses and fair value - (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012	Dec. 31, 2011
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	$ 14,358,761,000	[1]	$ 14,399,284,000	[1]
Gross unrealized gains	731,950,000		621,505,000
Gross unrealized losses	(89,774,000)		(192,887,000)
Fair value	15,000,937,000		14,827,902,000
Cash and cash equivalents	1,313,468,000		1,496,485,000		1,121,705,000	1,342,257,000
Accrued investment income	158,737,000		185,717,000
Funds held directly managed [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	464,273,000		554,275,000	[2]
Gross unrealized gains	20,488,000		21,497,000
Gross unrealized losses	(12,015,000)		(14,327,000)
Fair value	472,746,000		561,445,000
Cash and cash equivalents	42,300,000		84,800,000
Other Assets Funds Held Directly Managed	88,200,000		132,900,000
Accrued investment income	5,700,000		6,700,000
Funds held directly managed [Member] | Fixed maturities
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	438,885,000		523,758,000	[2]
Gross unrealized gains	20,488,000		21,497,000
Gross unrealized losses	(178,000)		(1,540,000)
Fair value	459,195,000		543,715,000
Funds held directly managed [Member] | Fixed maturities | U.S. government and government sponsored enterprises
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	150,242,000		153,951,000	[2]
Gross unrealized gains	3,302,000		3,789,000
Gross unrealized losses	(61,000)		(444,000)
Fair value	153,483,000		157,296,000
Funds held directly managed [Member] | Fixed maturities | U.S. states, territories and municipalities
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	214,000		372,000	[2]
Gross unrealized gains	0		0
Gross unrealized losses	(82,000)		(86,000)
Fair value	132,000		286,000
Funds held directly managed [Member] | Fixed maturities | Non-U.S. sovereign government, supranational and government related
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	119,732,000		131,488,000	[2]
Gross unrealized gains	8,536,000		6,708,000
Gross unrealized losses	(35,000)		(1,010,000)
Fair value	128,233,000		137,186,000
Funds held directly managed [Member] | Fixed maturities | Corporate
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	168,697,000		237,947,000	[2]
Gross unrealized gains	8,650,000		11,000,000
Gross unrealized losses	0		0
Fair value	177,347,000		248,947,000
Funds held directly managed [Member] | Short-term investments
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost			2,426,000	[2]
Gross unrealized gains			0
Gross unrealized losses			0
Fair value			2,426,000
Funds held directly managed [Member] | Other invested assets funds held directly managed [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	25,388,000		28,091,000	[2]
Gross unrealized gains	0		0
Gross unrealized losses	(11,837,000)		(12,787,000)
Fair value	$ 13,551,000		$ 15,304,000

[1] Cost is amortized cost for fixed maturities and short-term investments and cost for equity securities.

[2] (1)Cost is amortized cost for fixed maturities and short-term investments.