Investments - Analysis of continual unrealized loss position (Details) - Excluding Life Funds Withheld Assets - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Total fixed maturities and short-term investments AFS
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	$ 10,056,125	$ 2,896,210
Less than 12 months Gross Unrealized Losses	(147,738)	(61,772)
Equal to or greater than 12 months Fair Value	1,734,541	2,745,705
Equal to greater than 12 months Gross Unrealized Losses	(108,226)	(125,574)
U.S. Government and Government - Related/Supported
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	1,721,385	251,091
Less than 12 months Gross Unrealized Losses	(12,211)	(1,196)
Equal to or greater than 12 months Fair Value	109,027	342,890
Equal to greater than 12 months Gross Unrealized Losses	(4,666)	(5,603)
RMBS - Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	1,136,559	134,535
Less than 12 months Gross Unrealized Losses	(12,171)	(220)
Equal to or greater than 12 months Fair Value	245,806	512,652
Equal to greater than 12 months Gross Unrealized Losses	(8,649)	(10,563)
RMBS - Non-Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	45,052	45,378
Less than 12 months Gross Unrealized Losses	(1,959)	(1,358)
Equal to or greater than 12 months Fair Value	197,361	202,700
Equal to greater than 12 months Gross Unrealized Losses	(15,962)	(16,797)
Commercial mortgage-backed securities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	295,914	78,356
Less than 12 months Gross Unrealized Losses	(5,263)	(385)
Equal to or greater than 12 months Fair Value	100,100	169,065
Equal to greater than 12 months Gross Unrealized Losses	(3,048)	(4,877)
Collateralized debt obligations
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	39,969	249,803
Less than 12 months Gross Unrealized Losses	(5,506)	(2,666)
Equal to or greater than 12 months Fair Value	322,684	414,516
Equal to greater than 12 months Gross Unrealized Losses	(19,067)	(24,503)
Other asset-backed securities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	1,014,257	143,044
Less than 12 months Gross Unrealized Losses	(6,535)	(2,813)
Equal to or greater than 12 months Fair Value	59,985	57,544
Equal to greater than 12 months Gross Unrealized Losses	(5,392)	(3,232)
US States and political subdivisions of the States
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	706,360	32,187
Less than 12 months Gross Unrealized Losses	(10,617)	(210)
Equal to or greater than 12 months Fair Value	17,546	63,695
Equal to greater than 12 months Gross Unrealized Losses	(1,181)	(994)
Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	1,736,079	624,346
Less than 12 months Gross Unrealized Losses	(38,213)	(19,043)
Equal to or greater than 12 months Fair Value	459,362	558,422
Equal to greater than 12 months Gross Unrealized Losses	(33,833)	(43,251)
Equities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	267,759	191,193
Less than 12 months Gross Unrealized Losses	(26,787)	(26,230)
Equal to or greater than 12 months Fair Value	0	0
Equal to greater than 12 months Gross Unrealized Losses	0	0
Financial Services Sector [Member] | Corporate
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	1,054,854	387,619
Less than 12 months Gross Unrealized Losses	(13,737)	(5,858)
Equal to or greater than 12 months Fair Value	76,450	105,155
Equal to greater than 12 months Gross Unrealized Losses	(6,846)	(7,097)
Non-Financial Services Sector [Member] | Corporate
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	2,305,696	949,851
Less than 12 months Gross Unrealized Losses	(41,526)	(28,023)
Equal to or greater than 12 months Fair Value	146,220	319,066
Equal to greater than 12 months Gross Unrealized Losses	$ (9,582)	$ (8,657)