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, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		$ 29,587,656	$ 31,517,654
Fixed maturities, AFS, at fair value		30,256,602	32,257,589
Amortized cost of short-term investments, available for sale		626,142	618,851
Short-term investments, at fair value		625,193	617,390
Cost of equity securities, available for sale		996,610	834,079
Equity securities, AFS, at fair value		1,037,331	878,919
Amortized cost		31,210,408	32,970,584
Gross unrealized gains		1,078,735	1,153,289
Gross unrealized losses		(370,017)	(369,975)
Total investments available for sale		31,919,126	33,753,898
Non credit-related OTTI	[1]	(51,135)	(57,734)
Exclude Life Funds Withheld Assets
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		27,768,929	28,839,622
Gross unrealized gains, fixed maturities		540,262	587,258
Gross unrealized losses, fixed maturities		(360,631)	(322,394)
Fixed maturities, AFS, at fair value		27,948,560	29,104,486
Amortized cost of short-term investments, available for sale		626,142	618,851
Gross unrealized gains, short term investments		227	967
Gross unrealized losses, short term investments		(1,176)	(2,428)
Short-term investments, at fair value		625,193	617,390
Cost of equity securities, available for sale		996,610	834,079
Gross unrealized gains, equities		48,931	89,993
Gross unrealized losses, equities		(8,210)	(45,153)
Equity securities, AFS, at fair value		1,037,331	878,919
Amortized cost		29,391,681	30,292,552
Gross unrealized gains		589,420	678,218
Gross unrealized losses		(370,017)	(369,975)
Total investments available for sale		29,611,084	30,600,795
Non credit-related OTTI	[1]	(51,135)	(57,734)
Exclude Life Funds Withheld Assets | U.S. Government and Government - Related/Supported
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		3,874,038	5,047,621
Gross unrealized gains, fixed maturities		53,172	52,355
Gross unrealized losses, fixed maturities		(32,822)	(34,339)
Fixed maturities, AFS, at fair value		3,894,388	5,065,637
Non credit-related OTTI	[1]	0	0
Exclude Life Funds Withheld Assets | 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		4,213,845	3,697,756
Gross unrealized gains, fixed maturities		49,913	77,776
Gross unrealized losses, fixed maturities		(45,391)	(17,561)
Fixed maturities, AFS, at fair value		4,218,367	3,757,971
Non credit-related OTTI	[1]	0	0
Exclude Life Funds Withheld Assets | RMBS - Non-Agency
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		265,877	319,876
Gross unrealized gains, fixed maturities		21,132	25,644
Gross unrealized losses, fixed maturities		(12,751)	(16,980)
Fixed maturities, AFS, at fair value		274,258	328,540
Non credit-related OTTI	[1]	(47,879)	(54,200)
Exclude Life Funds Withheld Assets | CMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		670,005	401,713
Gross unrealized gains, fixed maturities		5,955	7,933
Gross unrealized losses, fixed maturities		(10,774)	(4,330)
Fixed maturities, AFS, at fair value		665,186	405,316
Non credit-related OTTI	[1]	(1,191)	(1,182)
Exclude Life Funds Withheld Assets | Collateralized debt obligations ('CDO')
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		379,532	41,679
Gross unrealized gains, fixed maturities		498	4
Gross unrealized losses, fixed maturities		(2,539)	(9,273)
Fixed maturities, AFS, at fair value		377,491	32,410
Non credit-related OTTI	[1]	(1,208)	(1,208)
Exclude Life Funds Withheld Assets | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		872,903	1,164,426
Gross unrealized gains, fixed maturities		12,697	17,665
Gross unrealized losses, fixed maturities		(9,703)	(13,519)
Fixed maturities, AFS, at fair value		875,897	1,168,572
Non credit-related OTTI	[1]	(821)	(1,144)
Exclude Life Funds Withheld Assets | U.S. States and political subdivisions of the States
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		2,399,490	2,514,048
Gross unrealized gains, fixed maturities		86,041	125,395
Gross unrealized losses, fixed maturities		(7,419)	(7,373)
Fixed maturities, AFS, at fair value		2,478,112	2,632,070
Non credit-related OTTI	[1]	0	0
Exclude Life Funds Withheld Assets | Non-U.S. Government
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		5,037,482	5,249,148
Gross unrealized gains, fixed maturities		112,772	100,383
Gross unrealized losses, fixed maturities		(120,122)	(97,917)
Fixed maturities, AFS, at fair value		5,030,132	5,251,614
Non credit-related OTTI	[1]	0	0
Exclude Life Funds Withheld Assets | Financial Services Sector | Corporate
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		3,587,557	3,535,830
Gross unrealized gains, fixed maturities		59,968	49,535
Gross unrealized losses, fixed maturities		(25,235)	(23,456)
Fixed maturities, AFS, at fair value		3,622,290	3,561,909
Non credit-related OTTI	[1]	0	0
Exclude Life Funds Withheld Assets | Non-Financial Services Sector | Corporate
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		6,468,200	6,867,525
Gross unrealized gains, fixed maturities		138,114	130,568
Gross unrealized losses, fixed maturities		(93,875)	(97,646)
Fixed maturities, AFS, at fair value		6,512,439	6,900,447
Non credit-related OTTI	[1]	(36)	0
Life Funds Withheld Assets
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		1,818,727	2,678,032
Gross unrealized gains, fixed maturities		489,315	475,071
Gross unrealized losses, fixed maturities		0	0
Fixed maturities, AFS, at fair value		2,308,042	3,153,103
Non credit-related OTTI	[1]	0	0
Life Funds Withheld Assets | U.S. Government and Government - Related/Supported
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		8,468	10,721
Gross unrealized gains, fixed maturities		2,109	2,021
Gross unrealized losses, fixed maturities		0	0
Fixed maturities, AFS, at fair value		10,577	12,742
Non credit-related OTTI	[1]	0	0
Life Funds Withheld Assets | 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		470	591
Gross unrealized gains, fixed maturities		143	161
Gross unrealized losses, fixed maturities		0	0
Fixed maturities, AFS, at fair value		613	752
Non credit-related OTTI	[1]	0	0
Life Funds Withheld Assets | RMBS - Non-Agency
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		19,121	24,401
Gross unrealized gains, fixed maturities		2,646	2,552
Gross unrealized losses, fixed maturities		0	0
Fixed maturities, AFS, at fair value		21,767	26,953
Non credit-related OTTI	[1]	0	0
Life Funds Withheld Assets | CMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		77,762	107,968
Gross unrealized gains, fixed maturities		16,478	14,513
Gross unrealized losses, fixed maturities		0	0
Fixed maturities, AFS, at fair value		94,240	122,481
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, available for sale		84,541	132,674
Gross unrealized gains, fixed maturities		17,019	17,121
Gross unrealized losses, fixed maturities		0	0
Fixed maturities, AFS, at fair value		101,560	149,795
Non credit-related OTTI	[1]	0	0
Life Funds Withheld Assets | Non-U.S. Government
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		454,695	737,735
Gross unrealized gains, fixed maturities		169,982	195,781
Gross unrealized losses, fixed maturities		0	0
Fixed maturities, AFS, at fair value		624,677	933,516
Non credit-related OTTI	[1]	0	0
Life Funds Withheld Assets | Financial Services Sector | Corporate
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		376,018	531,016
Gross unrealized gains, fixed maturities		63,249	67,220
Gross unrealized losses, fixed maturities		0	0
Fixed maturities, AFS, at fair value		439,267	598,236
Non credit-related OTTI	[1]	0	0
Life Funds Withheld Assets | Non-Financial Services Sector | Corporate
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities, available for sale		797,652	1,132,926
Gross unrealized gains, fixed maturities		217,689	175,702
Gross unrealized losses, fixed maturities		0	0
Fixed maturities, AFS, at fair value		1,015,341	1,308,628
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.