Investments Cost, unrealized gains and losses and fair value - (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	$ 14,358,761	[1]	$ 14,399,284	[1]
Gross unrealized gains	731,950		621,505
Gross unrealized losses	(89,774)		(192,887)
Fair value	15,000,937		14,827,902
Trading securities [Member] | Fixed maturities
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	13,489,633	[1]	13,376,455	[1]
Gross unrealized gains	491,279		371,213
Gross unrealized losses	(62,167)		(154,365)
Fair value	13,918,745		13,593,303
Trading securities [Member] | Fixed maturities | U.S. government and government sponsored enterprises
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	2,308,264	[1]	1,635,578	[1]
Gross unrealized gains	13,350		7,211
Gross unrealized losses	(6,192)		(18,930)
Fair value	2,315,422		1,623,859
Trading securities [Member] | Fixed maturities | U.S. states, territories and municipalities
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	511,228	[1]	121,697	[1]
Gross unrealized gains	21,058		4,395
Gross unrealized losses	(1,683)		(1,505)
Fair value	530,603		124,587
Trading securities [Member] | Fixed maturities | Non-U.S. sovereign government, supranational and government related
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	1,866,915	[1]	2,295,608	[1]
Gross unrealized gains	112,029		67,453
Gross unrealized losses	(2,742)		(9,362)
Fair value	1,976,202		2,353,699
Trading securities [Member] | Fixed maturities | Corporate
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	5,363,006	[1]	5,866,991	[1]
Gross unrealized gains	263,349		243,522
Gross unrealized losses	(22,195)		(61,850)
Fair value	5,604,160		6,048,663
Trading securities [Member] | Fixed maturities | Asset-backed securities [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	1,110,393	[1]	1,126,812	[1]
Gross unrealized gains	23,131		15,232
Gross unrealized losses	(2,104)		(3,813)
Fair value	1,131,420		1,138,231
Trading securities [Member] | Fixed maturities | Residential mortgage-backed securities
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	2,276,200	[1]	2,294,870	[1]
Gross unrealized gains	56,875		31,810
Gross unrealized losses	(26,599)		(58,163)
Fair value	2,306,476		2,268,517
Trading securities [Member] | Fixed maturities | Other mortgage-backed securities
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	53,627	[1]	34,899	[1]
Gross unrealized gains	1,487		1,590
Gross unrealized losses	(652)		(742)
Fair value	54,462		35,747
Trading securities [Member] | Short-term investments
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	25,699	[1]	13,543	[1]
Gross unrealized gains	4		4
Gross unrealized losses	(25)		(1)
Fair value	25,678		13,546
Trading securities [Member] | Equities
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost	843,429	[1]	1,009,286	[1]
Gross unrealized gains	240,667		250,288
Gross unrealized losses	(27,582)		(38,521)
Fair value	$ 1,056,514		$ 1,221,053

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