Investments - Analysis of continual unrealized loss position (Details) - Exclude Life Funds Withheld Assets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fixed maturities and short term investments
Fixed maturities and short-term investments - AFS: [Abstract]
Less than 12 months - fair value	$ 10,300,927	$ 11,830,367
Less than 12 months - gross unrealized losses	(82,110)	(249,365)
Equal to or greater than 12 months - fair value	3,089,852	951,580
Equal to or greater than 12 months - gross unrealized losses	(111,973)	(112,442)
U.S. Government
Fixed maturities and short-term investments - AFS: [Abstract]
Less than 12 months - fair value	2,687,083	2,456,685
Less than 12 months - gross unrealized losses	(21,514)	(30,640)
Equal to or greater than 12 months - fair value	727,246	43,873
Equal to or greater than 12 months - gross unrealized losses	(13,902)	(2,237)
U.S. States and political subdivisions of the States
Fixed maturities and short-term investments - AFS: [Abstract]
Less than 12 months - fair value	88,235	457,288
Less than 12 months - gross unrealized losses	(622)	(6,983)
Equal to or greater than 12 months - fair value	19,583	4,287
Equal to or greater than 12 months - gross unrealized losses	(366)	(437)
Non-U.S. Governments
Fixed maturities and short-term investments - AFS: [Abstract]
Less than 12 months - fair value	1,527,323	1,797,405
Less than 12 months - gross unrealized losses	(14,959)	(75,596)
Equal to or greater than 12 months - fair value	560,648	288,490
Equal to or greater than 12 months - gross unrealized losses	(33,931)	(45,480)
Corporate
Fixed maturities and short-term investments - AFS: [Abstract]
Less than 12 months - fair value	2,959,416	3,414,252
Less than 12 months - gross unrealized losses	(25,757)	(81,594)
Equal to or greater than 12 months - fair value	543,973	264,920
Equal to or greater than 12 months - gross unrealized losses	(31,577)	(37,595)
RMBS
Fixed maturities and short-term investments - AFS: [Abstract]
Less than 12 months - fair value	2,198,391	2,459,545
Less than 12 months - gross unrealized losses	(14,030)	(41,194)
Equal to or greater than 12 months - fair value	925,191	302,810
Equal to or greater than 12 months - gross unrealized losses	(21,093)	(16,948)
CMBS
Fixed maturities and short-term investments - AFS: [Abstract]
Less than 12 months - fair value	583,656	467,757
Less than 12 months - gross unrealized losses	(4,117)	(9,470)
Equal to or greater than 12 months - fair value	138,065	9,665
Equal to or greater than 12 months - gross unrealized losses	(5,740)	(1,304)
Other asset-backed securities
Fixed maturities and short-term investments - AFS: [Abstract]
Less than 12 months - fair value	256,823	777,435
Less than 12 months - gross unrealized losses	(1,111)	(3,888)
Equal to or greater than 12 months - fair value	175,146	37,535
Equal to or greater than 12 months - gross unrealized losses	(5,364)	(8,441)
Equity securities
Fixed maturities and short-term investments - AFS: [Abstract]
Less than 12 months - fair value	58,632	130,487
Less than 12 months - gross unrealized losses	(5,167)	(8,210)
Equal to or greater than 12 months - fair value	0	0
Equal to or greater than 12 months - gross unrealized losses	$ 0	$ 0