Investments - Analysis of continual unrealized loss position (Details) - Excluding Life Funds Withheld Assets - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Total fixed maturities and short-term investments AFS
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	$ 9,184,317	$ 11,830,367
Less than 12 months Gross Unrealized Losses	(67,412)	(249,365)
Equal to or greater than 12 months Fair Value	2,205,993	951,580
Equal to greater than 12 months Gross Unrealized Losses	(107,184)	(112,442)
U.S. Government and Government Agencies
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	2,744,113	2,456,685
Less than 12 months Gross Unrealized Losses	(14,476)	(30,640)
Equal to or greater than 12 months Fair Value	319,287	43,873
Equal to greater than 12 months Gross Unrealized Losses	(5,938)	(2,237)
US States, municipalities and political subdivisions of the States
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	116,193	457,288
Less than 12 months Gross Unrealized Losses	(690)	(6,983)
Equal to or greater than 12 months Fair Value	13,207	4,287
Equal to greater than 12 months Gross Unrealized Losses	(174)	(437)
Non-U.S. Governments
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	1,458,435	1,797,405
Less than 12 months Gross Unrealized Losses	(18,482)	(75,596)
Equal to or greater than 12 months Fair Value	654,433	288,490
Equal to greater than 12 months Gross Unrealized Losses	(43,097)	(45,480)
Corporate
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	1,860,565	3,414,252
Less than 12 months Gross Unrealized Losses	(16,568)	(81,594)
Equal to or greater than 12 months Fair Value	578,271	264,920
Equal to greater than 12 months Gross Unrealized Losses	(36,706)	(37,595)
Residential mortgage-backed securities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	2,079,510	2,459,545
Less than 12 months Gross Unrealized Losses	(12,193)	(41,194)
Equal to or greater than 12 months Fair Value	412,261	302,810
Equal to greater than 12 months Gross Unrealized Losses	(10,179)	(16,948)
Commercial mortgage-backed securities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	569,284	467,757
Less than 12 months Gross Unrealized Losses	(3,959)	(9,470)
Equal to or greater than 12 months Fair Value	109,574	9,665
Equal to greater than 12 months Gross Unrealized Losses	(5,082)	(1,304)
Other asset-backed securities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	356,217	777,435
Less than 12 months Gross Unrealized Losses	(1,044)	(3,888)
Equal to or greater than 12 months Fair Value	118,960	37,535
Equal to greater than 12 months Gross Unrealized Losses	(6,008)	(8,441)
Equities
Fixed maturities and short-term investments - AFS:
Less than 12 months Fair Value	38,170	130,487
Less than 12 months Gross Unrealized Losses	(2,677)	(8,210)
Equal to or greater than 12 months Fair Value	0	0
Equal to greater than 12 months Gross Unrealized Losses	$ 0	$ 0