Investments - Analysis of continual unrealized loss position (Details) - Exclude Life Funds Withheld Assets - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
U.S. Government
Fixed maturities and short-term investments - AFS: [Abstract]
Less than 12 months - fair value	$ 2,456,685	$ 3,762,869
Less than 12 months - gross unrealized losses	(30,640)	(29,339)
Equal to or greater than 12 months - fair value	43,873	89,113
Equal to or greater than 12 months - gross unrealized losses	(2,237)	(5,044)
Corporate | Financial Services Sector
Fixed maturities and short-term investments - AFS: [Abstract]
Less than 12 months - fair value	1,111,700	1,641,021
Less than 12 months - gross unrealized losses	(19,010)	(13,280)
Equal to or greater than 12 months - fair value	59,091	102,022
Equal to or greater than 12 months - gross unrealized losses	(6,227)	(10,192)
Corporate | Non-Financial Services Sector
Fixed maturities and short-term investments - AFS: [Abstract]
Less than 12 months - fair value	2,302,552	3,275,270
Less than 12 months - gross unrealized losses	(62,584)	(73,069)
Equal to or greater than 12 months - fair value	205,829	227,527
Equal to or greater than 12 months - gross unrealized losses	(31,368)	(24,706)
RMBS - Agency
Fixed maturities and short-term investments - AFS: [Abstract]
Less than 12 months - fair value	2,456,028	1,065,055
Less than 12 months - gross unrealized losses	(40,989)	(10,046)
Equal to or greater than 12 months - fair value	136,014	221,211
Equal to or greater than 12 months - gross unrealized losses	(4,402)	(7,515)
RMBS - Non-Agency
Fixed maturities and short-term investments - AFS: [Abstract]
Less than 12 months - fair value	3,517	19,614
Less than 12 months - gross unrealized losses	(205)	(1,104)
Equal to or greater than 12 months - fair value	166,796	180,146
Equal to or greater than 12 months - gross unrealized losses	(12,546)	(15,876)
CMBS
Fixed maturities and short-term investments - AFS: [Abstract]
Less than 12 months - fair value	467,757	118,605
Less than 12 months - gross unrealized losses	(9,470)	(1,561)
Equal to or greater than 12 months - fair value	9,665	78,651
Equal to or greater than 12 months - gross unrealized losses	(1,304)	(2,769)
Collateralized debt obligations ('CDO')
Fixed maturities and short-term investments - AFS: [Abstract]
Less than 12 months - fair value	105,653	12,311
Less than 12 months - gross unrealized losses	(58)	(516)
Equal to or greater than 12 months - fair value	2,752	20,096
Equal to or greater than 12 months - gross unrealized losses	(2,481)	(8,757)
Other asset-backed securities
Fixed maturities and short-term investments - AFS: [Abstract]
Less than 12 months - fair value	671,782	572,671
Less than 12 months - gross unrealized losses	(3,830)	(5,252)
Equal to or greater than 12 months - fair value	34,783	57,563
Equal to or greater than 12 months - gross unrealized losses	(5,960)	(8,268)
U.S. States and political subdivisions of the States
Fixed maturities and short-term investments - AFS: [Abstract]
Less than 12 months - fair value	457,288	565,055
Less than 12 months - gross unrealized losses	(6,983)	(6,609)
Equal to or greater than 12 months - fair value	4,287	12,259
Equal to or greater than 12 months - gross unrealized losses	(437)	(765)
Non-U.S. Government
Fixed maturities and short-term investments - AFS: [Abstract]
Less than 12 months - fair value	1,797,405	1,921,286
Less than 12 months - gross unrealized losses	(75,596)	(53,440)
Equal to or greater than 12 months - fair value	288,490	474,929
Equal to or greater than 12 months - gross unrealized losses	(45,480)	(46,714)
Fixed maturities and short term investments
Fixed maturities and short-term investments - AFS: [Abstract]
Less than 12 months - fair value	11,830,367	12,953,757
Less than 12 months - gross unrealized losses	(249,365)	(194,216)
Equal to or greater than 12 months - fair value	951,580	1,463,517
Equal to or greater than 12 months - gross unrealized losses	(112,442)	(130,606)
Equity securities
Fixed maturities and short-term investments - AFS: [Abstract]
Less than 12 months - fair value	130,487	356,742
Less than 12 months - gross unrealized losses	(8,210)	(45,153)
Equal to or greater than 12 months - fair value	0	0
Equal to or greater than 12 months - gross unrealized losses	$ 0	$ 0