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		Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		$ 30,268,346	$ 30,157,581
Fixed maturities AFS		30,656,628	30,928,988
Non-credit Related OTTI	[1]	(39,934)	(43,646)
Amortized cost of short-term investments, available for sale		500,844	816,638
Short-term investments, at fair value		500,170	815,481
Available-for-sale Securities, Amortized Cost Basis		30,769,190	31,612,674
Available for sale securities, Gross unrealized gains		787,275	1,045,012
Available for sale securities, Gross unrealized losses		(399,667)	(199,250)
Total investments available for sale		31,156,798	32,458,436
Excluding Life Funds Withheld Assets
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		28,668,435	28,569,530
Gross unrealized gains on Fixed maturities		377,865	524,528
Gross unrealized losses on Fixed maturities		(398,220)	(192,182)
Fixed maturities AFS		28,648,080	28,901,876
Non-credit Related OTTI	[1]	(39,934)	(43,646)
Amortized cost of short-term investments, available for sale		500,844	816,638
Short term investments, gross unrealized gains		773	744
Short term investments, gross unrealized losses		(1,447)	(1,901)
Short-term investments, at fair value		500,170	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		29,169,279	30,024,623
Available for sale securities, Gross unrealized gains		378,638	605,951
Available for sale securities, Gross unrealized losses		(399,667)	(199,250)
Total investments available for sale		29,148,250	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		4,207,380	4,358,503
Gross unrealized gains on Fixed maturities		29,290	37,782
Gross unrealized losses on Fixed maturities		(48,711)	(33,545)
Fixed maturities AFS		4,187,959	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		1,953,202	1,977,796
Gross unrealized gains on Fixed maturities		63,586	87,832
Gross unrealized losses on Fixed maturities		(3,626)	(988)
Fixed maturities AFS		2,013,162	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		5,260,650	5,135,526
Gross unrealized gains on Fixed maturities		98,496	114,918
Gross unrealized losses on Fixed maturities		(52,495)	(48,863)
Fixed maturities AFS		5,306,651	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		9,759,732	9,644,799
Gross unrealized gains on Fixed maturities		131,175	207,668
Gross unrealized losses on Fixed maturities		(148,237)	(57,334)
Fixed maturities AFS		9,742,670	9,795,133
Non-credit Related OTTI	[1]	0	(18)
Excluding Life Funds Withheld Assets | Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		4,606,275	4,717,542
Gross unrealized gains on Fixed maturities		26,883	41,983
Gross unrealized losses on Fixed maturities		(103,435)	(35,123)
Fixed maturities AFS		4,529,723	4,724,402
Non-credit Related OTTI	[1]	(8,434)	(8,795)
Excluding Life Funds Withheld Assets | Commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		1,292,559	1,246,406
Gross unrealized gains on Fixed maturities		2,263	7,468
Gross unrealized losses on Fixed maturities		(35,303)	(9,857)
Fixed maturities AFS		1,259,519	1,244,017
Non-credit Related OTTI	[1]	(736)	(817)
Excluding Life Funds Withheld Assets | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		1,588,637	1,488,958
Gross unrealized gains on Fixed maturities		26,172	26,877
Gross unrealized losses on Fixed maturities		(6,413)	(6,472)
Fixed maturities AFS		1,608,396	1,509,363
Non-credit Related OTTI	[1]	(30,764)	(34,016)
Life Funds Withheld Assets
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		1,599,911	1,588,051
Gross unrealized gains on Fixed maturities		408,637	439,061
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities AFS		2,008,548	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		9,420	9,050
Gross unrealized gains on Fixed maturities		2,196	2,266
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities AFS		11,616	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		432,390	433,664
Gross unrealized gains on Fixed maturities		152,016	150,870
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities AFS		584,406	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		919,151	909,589
Gross unrealized gains on Fixed maturities		202,052	227,624
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities AFS		1,121,203	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		455	448
Gross unrealized gains on Fixed maturities		48	67
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities AFS		503	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		105,505	97,356
Gross unrealized gains on Fixed maturities		24,565	24,916
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities AFS		130,070	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		132,990	137,944
Gross unrealized gains on Fixed maturities		27,760	33,318
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities AFS		160,750	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.