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		Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		$ 31,517,654	$ 27,728,771
Fixed maturities, AFS, at fair value		32,257,589	29,359,034
Amortized cost of short-term investments, available for sale		618,851	257,221
Short-term investments, at fair value		617,390	256,727
Cost of equity securities, available for sale		834,079	763,833
Equity securities, AFS, at fair value		878,919	868,292
Amortized cost		32,970,584	28,749,825
Gross unrealized gains		1,153,289	1,947,804
Gross unrealized losses		(369,975)	(213,576)
Fair value		33,753,898	30,484,053
Non credit-related OTTI	[1]	(57,734)	(76,720)
Exclude Life Funds Withheld Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		28,839,622	23,427,432
Gross unrealized gains, fixed maturities		587,258	936,840
Gross unrealized losses, fixed maturities		(322,394)	(186,803)
Fixed maturities, AFS, at fair value		29,104,486	24,177,469
Amortized cost of short-term investments, available for sale		618,851	257,221
Gross unrealized gains, short term investments		967	49
Gross unrealized losses, short term investments		(2,428)	(543)
Short-term investments, at fair value		617,390	256,727
Cost of equity securities, available for sale		834,079	763,833
Gross unrealized gains, equities		89,993	130,689
Gross unrealized losses, equities		(45,153)	(26,230)
Equity securities, AFS, at fair value		878,919	868,292
Amortized cost		30,292,552	24,448,486
Gross unrealized gains		678,218	1,067,578
Gross unrealized losses		(369,975)	(213,576)
Fair value		30,600,795	25,302,488
Non credit-related OTTI	[1]	(57,734)	(76,720)
Exclude Life Funds Withheld Assets [Member] | U.S. Government and Government - Related/Supported [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		5,047,621	2,100,851
Gross unrealized gains, fixed maturities		52,355	77,889
Gross unrealized losses, fixed maturities		(34,339)	(6,787)
Fixed maturities, AFS, at fair value		5,065,637	2,171,953
Non credit-related OTTI	[1]	0	0
Exclude Life Funds Withheld Assets [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		3,697,756	3,625,171
Gross unrealized gains, fixed maturities		77,776	114,188
Gross unrealized losses, fixed maturities		(17,561)	(10,783)
Fixed maturities, AFS, at fair value		3,757,971	3,728,576
Non credit-related OTTI	[1]	0	0
Exclude Life Funds Withheld Assets [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		319,876	404,398
Gross unrealized gains, fixed maturities		25,644	41,108
Gross unrealized losses, fixed maturities		(16,980)	(18,155)
Fixed maturities, AFS, at fair value		328,540	427,351
Non credit-related OTTI	[1]	(54,200)	(67,918)
Exclude Life Funds Withheld Assets [Member] | Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		401,713	1,033,819
Gross unrealized gains, fixed maturities		7,933	23,987
Gross unrealized losses, fixed maturities		(4,330)	(5,262)
Fixed maturities, AFS, at fair value		405,316	1,052,544
Non credit-related OTTI	[1]	(1,182)	(2,033)
Exclude Life Funds Withheld Assets [Member] | Collateralized debt obligations ('CDO') [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		41,679	717,544
Gross unrealized gains, fixed maturities		4	1,659
Gross unrealized losses, fixed maturities		(9,273)	(27,169)
Fixed maturities, AFS, at fair value		32,410	692,034
Non credit-related OTTI	[1]	(1,208)	(1,663)
Exclude Life Funds Withheld Assets [Member] | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		1,164,426	1,028,528
Gross unrealized gains, fixed maturities		17,665	42,810
Gross unrealized losses, fixed maturities		(13,519)	(6,045)
Fixed maturities, AFS, at fair value		1,168,572	1,065,293
Non credit-related OTTI	[1]	(1,144)	(1,797)
Exclude Life Funds Withheld Assets [Member] | U.S. States and political subdivisions of the States [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		2,514,048	1,892,566
Gross unrealized gains, fixed maturities		125,395	129,910
Gross unrealized losses, fixed maturities		(7,373)	(1,204)
Fixed maturities, AFS, at fair value		2,632,070	2,021,272
Non credit-related OTTI	[1]	0	0
Exclude Life Funds Withheld Assets [Member] | Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		5,249,148	4,162,425
Gross unrealized gains, fixed maturities		100,383	139,484
Gross unrealized losses, fixed maturities		(97,917)	(61,836)
Fixed maturities, AFS, at fair value		5,251,614	4,240,073
Non credit-related OTTI	[1]	0	0
Exclude Life Funds Withheld Assets [Member] | Financial Services Sector [Member] | Corporate
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		3,535,830	2,687,797
Gross unrealized gains, fixed maturities		49,535	87,058
Gross unrealized losses, fixed maturities		(23,456)	(12,939)
Fixed maturities, AFS, at fair value		3,561,909	2,761,916
Non credit-related OTTI	[1]	0	0
Exclude Life Funds Withheld Assets [Member] | Non-Financial Services Sector [Member] | Corporate
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		6,867,525	5,774,333
Gross unrealized gains, fixed maturities		130,568	278,747
Gross unrealized losses, fixed maturities		(97,646)	(36,623)
Fixed maturities, AFS, at fair value		6,900,447	6,016,457
Non credit-related OTTI	[1]	0	(3,309)
Life Funds Withheld Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		2,678,032	4,301,339
Gross unrealized gains, fixed maturities		475,071	880,226
Gross unrealized losses, fixed maturities		0	0
Fixed maturities, AFS, at fair value		3,153,103	5,181,565
Non credit-related OTTI	[1]	0	0
Life Funds Withheld Assets [Member] | U.S. Government and Government - Related/Supported [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		10,721	14,866
Gross unrealized gains, fixed maturities		2,021	3,858
Gross unrealized losses, fixed maturities		0	0
Fixed maturities, AFS, at fair value		12,742	18,724
Non credit-related OTTI	[1]	0	0
Life Funds Withheld Assets [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		591	3,301
Gross unrealized gains, fixed maturities		161	481
Gross unrealized losses, fixed maturities		0	0
Fixed maturities, AFS, at fair value		752	3,782
Non credit-related OTTI	[1]	0	0
Life Funds Withheld Assets [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		24,401	71,075
Gross unrealized gains, fixed maturities		2,552	14,260
Gross unrealized losses, fixed maturities		0	0
Fixed maturities, AFS, at fair value		26,953	85,335
Non credit-related OTTI	[1]	0	0
Life Funds Withheld Assets [Member] | Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		107,968	168,886
Gross unrealized gains, fixed maturities		14,513	24,281
Gross unrealized losses, fixed maturities		0	0
Fixed maturities, AFS, at fair value		122,481	193,167
Non credit-related OTTI	[1]	0	0
Life Funds Withheld Assets [Member] | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		132,674	238,168
Gross unrealized gains, fixed maturities		17,121	35,373
Gross unrealized losses, fixed maturities		0	0
Fixed maturities, AFS, at fair value		149,795	273,541
Non credit-related OTTI	[1]	0	0
Life Funds Withheld Assets [Member] | Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		737,735	1,397,194
Gross unrealized gains, fixed maturities		195,781	391,842
Gross unrealized losses, fixed maturities		0	0
Fixed maturities, AFS, at fair value		933,516	1,789,036
Non credit-related OTTI	[1]	0	0
Life Funds Withheld Assets [Member] | Financial Services Sector [Member] | Corporate
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		531,016	701,587
Gross unrealized gains, fixed maturities		67,220	99,432
Gross unrealized losses, fixed maturities		0	0
Fixed maturities, AFS, at fair value		598,236	801,019
Non credit-related OTTI	[1]	0	0
Life Funds Withheld Assets [Member] | Non-Financial Services Sector [Member] | Corporate
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		1,132,926	1,706,262
Gross unrealized gains, fixed maturities		175,702	310,699
Gross unrealized losses, fixed maturities		0	0
Fixed maturities, AFS, at fair value		1,308,628	2,016,961
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.