Investments - Analysis of continual unrealized loss position (Details) - Excluding Life Funds Withheld Assets - USD ($) $ in Thousands	Jun. 30, 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	$ 2,498,790	$ 12,953,757
Less than 12 months Gross Unrealized Losses	(98,010)	(194,216)
Equal to or greater than 12 months Fair Value	1,294,330	1,463,517
Equal to greater than 12 months Gross Unrealized Losses	(102,385)	(130,606)
U.S. Government and Government - Related/Supported
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	223,236	3,762,869
Less than 12 months Gross Unrealized Losses	(1,711)	(29,339)
Equal to or greater than 12 months Fair Value	49,239	89,113
Equal to greater than 12 months Gross Unrealized Losses	(1,541)	(5,044)
RMBS - Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	249,152	1,065,055
Less than 12 months Gross Unrealized Losses	(742)	(10,046)
Equal to or greater than 12 months Fair Value	211,310	221,211
Equal to greater than 12 months Gross Unrealized Losses	(1,658)	(7,515)
RMBS - Non-Agency
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	30,747	19,614
Less than 12 months Gross Unrealized Losses	(995)	(1,104)
Equal to or greater than 12 months Fair Value	172,556	180,146
Equal to greater than 12 months Gross Unrealized Losses	(17,071)	(15,876)
Commercial mortgage-backed securities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	19,125	118,605
Less than 12 months Gross Unrealized Losses	(789)	(1,561)
Equal to or greater than 12 months Fair Value	38,044	78,651
Equal to greater than 12 months Gross Unrealized Losses	(2,321)	(2,769)
Collateralized debt obligations
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	12,476	12,311
Less than 12 months Gross Unrealized Losses	(440)	(516)
Equal to or greater than 12 months Fair Value	5,317	20,096
Equal to greater than 12 months Gross Unrealized Losses	(3,200)	(8,757)
Other asset-backed securities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	285,791	572,671
Less than 12 months Gross Unrealized Losses	(1,296)	(5,252)
Equal to or greater than 12 months Fair Value	93,287	57,563
Equal to greater than 12 months Gross Unrealized Losses	(8,164)	(8,268)
US States and political subdivisions of the States
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	38,476	565,055
Less than 12 months Gross Unrealized Losses	(1,157)	(6,609)
Equal to or greater than 12 months Fair Value	10,152	12,259
Equal to greater than 12 months Gross Unrealized Losses	(101)	(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	856,460	1,921,286
Less than 12 months Gross Unrealized Losses	(43,696)	(53,440)
Equal to or greater than 12 months Fair Value	328,900	474,929
Equal to greater than 12 months Gross Unrealized Losses	(33,549)	(46,714)
Equities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	293,337	356,742
Less than 12 months Gross Unrealized Losses	(20,423)	(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	350,614	1,641,021
Less than 12 months Gross Unrealized Losses	(19,762)	(13,280)
Equal to or greater than 12 months Fair Value	115,843	102,022
Equal to greater than 12 months Gross Unrealized Losses	(7,573)	(10,192)
Non-Financial Services Sector | Corporate
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	432,713	3,275,270
Less than 12 months Gross Unrealized Losses	(27,422)	(73,069)
Equal to or greater than 12 months Fair Value	269,682	227,527
Equal to greater than 12 months Gross Unrealized Losses	$ (27,207)	$ (24,706)