Investments - Analysis of continual unrealized loss position (Details) - Excluding Life Funds Withheld Assets - USD ($) $ in Thousands	Sep. 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	$ 7,750,972	$ 2,896,210
Less than 12 months Gross Unrealized Losses	(136,075)	(61,772)
Equal to or greater than 12 months Fair Value	1,647,894	2,745,705
Equal to greater than 12 months Gross Unrealized Losses	(108,434)	(125,574)
U.S. Government and Government - Related/Supported
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	1,257,855	251,091
Less than 12 months Gross Unrealized Losses	(10,402)	(1,196)
Equal to or greater than 12 months Fair Value	89,971	342,890
Equal to greater than 12 months Gross Unrealized Losses	(4,524)	(5,603)
RMBS - Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	590,089	134,535
Less than 12 months Gross Unrealized Losses	(3,386)	(220)
Equal to or greater than 12 months Fair Value	237,798	512,652
Equal to greater than 12 months Gross Unrealized Losses	(5,205)	(10,563)
RMBS - Non-Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	28,054	45,378
Less than 12 months Gross Unrealized Losses	(1,022)	(1,358)
Equal to or greater than 12 months Fair Value	206,549	202,700
Equal to greater than 12 months Gross Unrealized Losses	(15,334)	(16,797)
Commercial mortgage-backed securities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	250,635	78,356
Less than 12 months Gross Unrealized Losses	(2,361)	(385)
Equal to or greater than 12 months Fair Value	87,257	169,065
Equal to greater than 12 months Gross Unrealized Losses	(2,613)	(4,877)
Collateralized debt obligations
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	118,002	249,803
Less than 12 months Gross Unrealized Losses	(286)	(2,666)
Equal to or greater than 12 months Fair Value	194,633	414,516
Equal to greater than 12 months Gross Unrealized Losses	(10,056)	(24,503)
Other asset-backed securities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	639,048	143,044
Less than 12 months Gross Unrealized Losses	(4,403)	(2,813)
Equal to or greater than 12 months Fair Value	57,445	57,544
Equal to greater than 12 months Gross Unrealized Losses	(6,342)	(3,232)
US States and political subdivisions of the States
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	444,195	32,187
Less than 12 months Gross Unrealized Losses	(5,520)	(210)
Equal to or greater than 12 months Fair Value	17,840	63,695
Equal to greater than 12 months Gross Unrealized Losses	(1,131)	(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	1,379,798	624,346
Less than 12 months Gross Unrealized Losses	(47,122)	(19,043)
Equal to or greater than 12 months Fair Value	454,537	558,422
Equal to greater than 12 months Gross Unrealized Losses	(34,683)	(43,251)
Equities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	524,692	191,193
Less than 12 months Gross Unrealized Losses	(81,272)	(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 | Corporate
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	862,043	387,619
Less than 12 months Gross Unrealized Losses	(10,486)	(5,858)
Equal to or greater than 12 months Fair Value	83,008	105,155
Equal to greater than 12 months Gross Unrealized Losses	(7,327)	(7,097)
Non-Financial Services Sector | Corporate
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	2,181,253	949,851
Less than 12 months Gross Unrealized Losses	(51,087)	(28,023)
Equal to or greater than 12 months Fair Value	218,856	319,066
Equal to greater than 12 months Gross Unrealized Losses	$ (21,219)	$ (8,657)