Investments - Cost (amortized cost for fixed maturities and short-term investments), fair value, gross unrealized gains, gross unrealized (losses), and OTTI recorded in AOCI (Details) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		$ 29,689,322	$ 30,157,581
Fixed maturities AFS		29,793,151	30,928,988
Non-credit Related OTTI	[1]	(37,936)	(43,646)
Amortized cost of short-term investments, available for sale		623,158	816,638
Short-term investments, at fair value		623,113	815,481
Available-for-sale Securities, Amortized Cost Basis		30,312,480	31,612,674
Available for sale securities, Gross unrealized gains		610,486	1,045,012
Available for sale securities, Gross unrealized losses		(506,702)	(199,250)
Total investments available for sale		30,416,264	32,458,436
Excluding Life Funds Withheld Assets
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		28,413,341	28,569,530
Gross unrealized gains on Fixed maturities		294,081	524,528
Gross unrealized losses on Fixed maturities		(506,005)	(192,182)
Fixed maturities AFS		28,201,417	28,901,876
Non-credit Related OTTI	[1]	(37,936)	(43,646)
Amortized cost of short-term investments, available for sale		623,158	816,638
Short term investments, gross unrealized gains		652	744
Short term investments, gross unrealized losses		(697)	(1,901)
Short-term investments, at fair value		623,113	815,481
Cost of equity securities			638,455
Equity securities, gross unrealized gains			80,679
Equity securities, gross unrealized losses			(5,167)
Equity securities, at fair value			713,967
Available-for-sale Securities, Amortized Cost Basis		29,036,499	30,024,623
Available for sale securities, Gross unrealized gains		294,733	605,951
Available for sale securities, Gross unrealized losses		(506,702)	(199,250)
Total investments available for sale		28,824,530	30,431,324
Excluding Life Funds Withheld Assets | U.S. Government and Government Agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		3,984,381	4,358,503
Gross unrealized gains on Fixed maturities		23,802	37,782
Gross unrealized losses on Fixed maturities		(47,892)	(33,545)
Fixed maturities AFS		3,960,291	4,362,740
Non-credit Related OTTI	[1]	0	0
Excluding Life Funds Withheld Assets | US States, municipalities and political subdivisions of the States
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		1,810,556	1,977,796
Gross unrealized gains on Fixed maturities		51,485	87,832
Gross unrealized losses on Fixed maturities		(3,899)	(988)
Fixed maturities AFS		1,858,142	2,064,640
Non-credit Related OTTI	[1]	0	0
Excluding Life Funds Withheld Assets | Non-U.S. Governments
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		5,117,027	5,135,526
Gross unrealized gains on Fixed maturities		74,297	114,918
Gross unrealized losses on Fixed maturities		(75,464)	(48,863)
Fixed maturities AFS		5,115,860	5,201,581
Non-credit Related OTTI	[1]	0	0
Excluding Life Funds Withheld Assets | Corporate
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		10,059,449	9,644,799
Gross unrealized gains on Fixed maturities		96,627	207,668
Gross unrealized losses on Fixed maturities		(205,981)	(57,334)
Fixed maturities AFS		9,950,095	9,795,133
Non-credit Related OTTI	[1]	(5)	(18)
Excluding Life Funds Withheld Assets | Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		4,348,536	4,717,542
Gross unrealized gains on Fixed maturities		22,531	41,983
Gross unrealized losses on Fixed maturities		(117,031)	(35,123)
Fixed maturities AFS		4,254,036	4,724,402
Non-credit Related OTTI	[1]	(8,158)	(8,795)
Excluding Life Funds Withheld Assets | Commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		1,410,363	1,246,406
Gross unrealized gains on Fixed maturities		1,901	7,468
Gross unrealized losses on Fixed maturities		(48,414)	(9,857)
Fixed maturities AFS		1,363,850	1,244,017
Non-credit Related OTTI	[1]	(406)	(817)
Excluding Life Funds Withheld Assets | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		1,683,029	1,488,958
Gross unrealized gains on Fixed maturities		23,438	26,877
Gross unrealized losses on Fixed maturities		(7,324)	(6,472)
Fixed maturities AFS		1,699,143	1,509,363
Non-credit Related OTTI	[1]	(29,367)	(34,016)
Life Funds Withheld Assets
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		1,275,981	1,588,051
Gross unrealized gains on Fixed maturities		315,753	439,061
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities AFS		1,591,734	2,027,112
Non-credit Related OTTI	[1]	0	0
Life Funds Withheld Assets | U.S. Government and Government Agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		8,801	9,050
Gross unrealized gains on Fixed maturities		2,133	2,266
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities AFS		10,934	11,316
Non-credit Related OTTI	[1]	0	0
Life Funds Withheld Assets | Non-U.S. Governments
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		347,074	433,664
Gross unrealized gains on Fixed maturities		120,846	150,870
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities AFS		467,920	584,534
Non-credit Related OTTI	[1]	0	0
Life Funds Withheld Assets | Corporate
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		719,866	909,589
Gross unrealized gains on Fixed maturities		149,728	227,624
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities AFS		869,594	1,137,213
Non-credit Related OTTI	[1]	0	0
Life Funds Withheld Assets | Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		412	448
Gross unrealized gains on Fixed maturities		71	67
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities AFS		483	515
Non-credit Related OTTI	[1]	0	0
Life Funds Withheld Assets | Commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		97,003	97,356
Gross unrealized gains on Fixed maturities		22,608	24,916
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities AFS		119,611	122,272
Non-credit Related OTTI	[1]	0	0
Life Funds Withheld Assets | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		102,825	137,944
Gross unrealized gains on Fixed maturities		20,367	33,318
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities AFS		123,192	171,262
Non-credit Related OTTI	[1]	$ 0	$ 0

[1] Represents the non-credit component of OTTI losses, adjusted for subsequent sales of securities. It does not include the change in fair value subsequent to the impairment measurement date.