Investments - Analysis of continual unrealized loss position (Details) - Excluding Life Funds Withheld Assets - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Total fixed maturities and short-term investments AFS
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	$ 3,555,794	$ 12,953,757
Less than 12 months Gross Unrealized Losses	(81,512)	(194,216)
Equal to or greater than 12 months Fair Value	1,540,883	1,463,517
Equal to greater than 12 months Gross Unrealized Losses	(111,526)	(130,606)
U.S. Government and Government - Related/Supported
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	465,423	3,762,869
Less than 12 months Gross Unrealized Losses	(4,923)	(29,339)
Equal to or greater than 12 months Fair Value	96,406	89,113
Equal to greater than 12 months Gross Unrealized Losses	(2,984)	(5,044)
RMBS - Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	151,135	1,065,055
Less than 12 months Gross Unrealized Losses	(1,128)	(10,046)
Equal to or greater than 12 months Fair Value	327,657	221,211
Equal to greater than 12 months Gross Unrealized Losses	(3,963)	(7,515)
RMBS - Non-Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	35,930	19,614
Less than 12 months Gross Unrealized Losses	(1,867)	(1,104)
Equal to or greater than 12 months Fair Value	175,881	180,146
Equal to greater than 12 months Gross Unrealized Losses	(18,980)	(15,876)
Commercial mortgage-backed securities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	31,661	118,605
Less than 12 months Gross Unrealized Losses	(255)	(1,561)
Equal to or greater than 12 months Fair Value	55,310	78,651
Equal to greater than 12 months Gross Unrealized Losses	(2,470)	(2,769)
Collateralized debt obligations
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	12,368	12,311
Less than 12 months Gross Unrealized Losses	(509)	(516)
Equal to or greater than 12 months Fair Value	9,359	20,096
Equal to greater than 12 months Gross Unrealized Losses	(3,609)	(8,757)
Other asset-backed securities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	282,395	572,671
Less than 12 months Gross Unrealized Losses	(2,113)	(5,252)
Equal to or greater than 12 months Fair Value	60,638	57,563
Equal to greater than 12 months Gross Unrealized Losses	(8,791)	(8,268)
US States and political subdivisions of the States
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	91,846	565,055
Less than 12 months Gross Unrealized Losses	(379)	(6,609)
Equal to or greater than 12 months Fair Value	15,528	12,259
Equal to greater than 12 months Gross Unrealized Losses	(414)	(765)
Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	965,976	1,921,286
Less than 12 months Gross Unrealized Losses	(22,116)	(53,440)
Equal to or greater than 12 months Fair Value	370,747	474,929
Equal to greater than 12 months Gross Unrealized Losses	(35,912)	(46,714)
Equities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	365,368	356,742
Less than 12 months Gross Unrealized Losses	(46,653)	(45,153)
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	588,595	1,641,021
Less than 12 months Gross Unrealized Losses	(18,228)	(13,280)
Equal to or greater than 12 months Fair Value	141,508	102,022
Equal to greater than 12 months Gross Unrealized Losses	(9,330)	(10,192)
Non-Financial Services Sector | Corporate
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	930,465	3,275,270
Less than 12 months Gross Unrealized Losses	(29,994)	(73,069)
Equal to or greater than 12 months Fair Value	287,849	227,527
Equal to greater than 12 months Gross Unrealized Losses	$ (25,073)	$ (24,706)