Funds Held - Directly Managed Cost, unrealized gains (losses) and fair value - (Details) - USD ($) $ in Thousands		Dec. 31, 2015	Dec. 31, 2014		Dec. 31, 2013	Dec. 31, 2012
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	[1]	$ 13,778,936	$ 14,358,761
Gross unrealized gains		380,754	731,950
Gross unrealized losses		(220,879)	(89,774)
Fair value		13,938,811	15,000,937
Cash and cash equivalents		1,577,097	1,313,468		$ 1,496,485	$ 1,121,705
Accrued investment income		141,672	158,737
Funds held directly managed [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost		397,843	464,273	[2]
Gross unrealized gains		13,660	20,488
Gross unrealized losses		(11,578)	(12,015)
Fair value		399,925	472,746
Cash and cash equivalents		64,600	42,300
Other Assets Funds Held Directly Managed		70,700	88,200
Accrued investment income		4,500	5,700
Funds held directly managed [Member] | Fixed maturities
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost		375,646	438,885	[2]
Gross unrealized gains		13,660	20,488
Gross unrealized losses		(519)	(178)
Fair value		388,787	459,195
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		168,197	150,242	[2]
Gross unrealized gains		2,209	3,302
Gross unrealized losses		(455)	(61)
Fair value		169,951	153,483
Funds held directly managed [Member] | Fixed maturities | U.S. states, territories and municipalities
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	[2]		214
Gross unrealized gains			0
Gross unrealized losses			(82)
Fair value			132
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		112,724	119,732	[2]
Gross unrealized gains		6,827	8,536
Gross unrealized losses		(64)	(35)
Fair value		119,487	128,233
Funds held directly managed [Member] | Fixed maturities | Corporate
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost		94,725	168,697	[2]
Gross unrealized gains		4,624	8,650
Gross unrealized losses		0	0
Fair value		99,349	177,347
Funds held directly managed [Member] | Short-term investments
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost		966
Gross unrealized gains		0
Gross unrealized losses		0
Fair value		966
Funds held directly managed [Member] | Other invested assets funds held directly managed [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost		21,231	25,388	[2]
Gross unrealized gains		0	0
Gross unrealized losses		(11,059)	(11,837)
Fair value		$ 10,172	$ 13,551

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

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