Investments - Analysis of continual unrealized loss position (Details) - Excluding Life Funds Withheld Assets - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Total fixed maturities and short-term investments AFS
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	$ 15,292,801	$ 10,300,927
Less than 12 months Gross Unrealized Losses	(279,115)	(82,110)
Equal to or greater than 12 months Fair Value	2,639,386	3,089,852
Equal to greater than 12 months Gross Unrealized Losses	(120,552)	(111,973)
U.S. Government and Government Agencies
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	3,080,855	2,687,083
Less than 12 months Gross Unrealized Losses	(38,758)	(21,514)
Equal to or greater than 12 months Fair Value	515,999	727,246
Equal to greater than 12 months Gross Unrealized Losses	(11,350)	(13,902)
US States, municipalities and political subdivisions of the States
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	278,946	88,235
Less than 12 months Gross Unrealized Losses	(3,222)	(622)
Equal to or greater than 12 months Fair Value	15,869	19,583
Equal to greater than 12 months Gross Unrealized Losses	(404)	(366)
Non-U.S. Governments
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	2,061,869	1,527,323
Less than 12 months Gross Unrealized Losses	(25,703)	(14,959)
Equal to or greater than 12 months Fair Value	487,103	560,648
Equal to greater than 12 months Gross Unrealized Losses	(26,836)	(33,931)
Corporate
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	5,509,770	2,959,416
Less than 12 months Gross Unrealized Losses	(114,092)	(25,757)
Equal to or greater than 12 months Fair Value	510,757	543,973
Equal to greater than 12 months Gross Unrealized Losses	(34,148)	(31,577)
Residential mortgage-backed securities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	2,973,309	2,198,391
Less than 12 months Gross Unrealized Losses	(68,157)	(14,030)
Equal to or greater than 12 months Fair Value	846,423	925,191
Equal to greater than 12 months Gross Unrealized Losses	(35,278)	(21,093)
Commercial mortgage-backed securities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	991,765	583,656
Less than 12 months Gross Unrealized Losses	(26,681)	(4,117)
Equal to or greater than 12 months Fair Value	131,116	138,065
Equal to greater than 12 months Gross Unrealized Losses	(8,622)	(5,740)
Other asset-backed securities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	396,287	256,823
Less than 12 months Gross Unrealized Losses	(2,502)	(1,111)
Equal to or greater than 12 months Fair Value	132,119	175,146
Equal to greater than 12 months Gross Unrealized Losses	$ (3,914)	$ (5,364)