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, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		$ 30,012,097	$ 29,587,656
Fixed maturities, at fair value		30,810,724	30,256,602
Non-credit Related OTTI	[1]	(48,218)	(51,135)
Amortized cost of short-term investments, available for sale		617,866	626,142
Short-term investments, at fair value		617,894	625,193
Cost of equity securities		1,094,494	996,610
Equity securities, at fair value		1,174,500	1,037,331
Available-for-sale Securities, Amortized Cost Basis		31,724,457	31,210,408
Available for sale securities, Gross unrealized gains		1,106,879	1,078,735
Available for sale securities, Gross unrealized losses		(228,218)	(370,017)
Total investments available for sale		32,603,118	31,919,126
Excluding Life Funds Withheld Assets
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		28,300,335	27,768,929
Gross unrealized gains on Fixed maturities		565,461	540,262
Gross unrealized losses on Fixed maturities		(223,857)	(360,631)
Fixed maturities, at fair value		28,641,939	27,948,560
Non-credit Related OTTI	[1]	(48,218)	(51,135)
Amortized cost of short-term investments, available for sale		617,866	626,142
Short term investments, gross unrealized gains		287	227
Short term investments, gross unrealized losses		(259)	(1,176)
Short-term investments, at fair value		617,894	625,193
Cost of equity securities		1,094,494	996,610
Equity securities, gross unrealized gains		84,108	48,931
Equity securities, gross unrealized losses		(4,102)	(8,210)
Equity securities, at fair value		1,174,500	1,037,331
Available-for-sale Securities, Amortized Cost Basis		30,012,695	29,391,681
Available for sale securities, Gross unrealized gains		649,856	589,420
Available for sale securities, Gross unrealized losses		(228,218)	(370,017)
Total investments available for sale		30,434,333	29,611,084
Excluding Life Funds Withheld Assets | U.S. Government and Government Agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		4,338,450	3,874,038
Gross unrealized gains on Fixed maturities		44,745	53,172
Gross unrealized losses on Fixed maturities		(27,794)	(32,822)
Fixed maturities, at fair value		4,355,401	3,894,388
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		2,180,145	2,399,490
Gross unrealized gains on Fixed maturities		105,214	86,041
Gross unrealized losses on Fixed maturities		(2,087)	(7,419)
Fixed maturities, at fair value		2,283,272	2,478,112
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		5,087,942	5,037,482
Gross unrealized gains on Fixed maturities		109,100	112,772
Gross unrealized losses on Fixed maturities		(73,518)	(120,122)
Fixed maturities, at fair value		5,123,524	5,030,132
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		10,391,238	10,055,757
Gross unrealized gains on Fixed maturities		219,988	198,082
Gross unrealized losses on Fixed maturities		(68,311)	(119,110)
Fixed maturities, at fair value		10,542,915	10,134,729
Non-credit Related OTTI	[1]	(18)	(36)
Excluding Life Funds Withheld Assets | Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		3,937,837	4,479,722
Gross unrealized gains on Fixed maturities		51,724	71,045
Gross unrealized losses on Fixed maturities		(35,243)	(58,142)
Fixed maturities, at fair value		3,954,318	4,492,625
Non-credit Related OTTI	[1]	(10,475)	(47,879)
Excluding Life Funds Withheld Assets | Commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		794,364	670,005
Gross unrealized gains on Fixed maturities		8,356	5,955
Gross unrealized losses on Fixed maturities		(7,267)	(10,774)
Fixed maturities, at fair value		795,453	665,186
Non-credit Related OTTI	[1]	(1,169)	(1,191)
Excluding Life Funds Withheld Assets | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		1,570,359	1,252,435
Gross unrealized gains on Fixed maturities		26,334	13,195
Gross unrealized losses on Fixed maturities		(9,637)	(12,242)
Fixed maturities, at fair value		1,587,056	1,253,388
Non-credit Related OTTI	[1]	(36,556)	(2,029)
Life Funds Withheld Assets
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		1,711,762	1,818,727
Gross unrealized gains on Fixed maturities		457,023	489,315
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities, at fair value		2,168,785	2,308,042
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		8,872	8,468
Gross unrealized gains on Fixed maturities		2,051	2,109
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities, at fair value		10,923	10,577
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		445,541	454,695
Gross unrealized gains on Fixed maturities		150,195	169,982
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities, at fair value		595,736	624,677
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		1,004,706	1,173,670
Gross unrealized gains on Fixed maturities		242,621	280,938
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities, at fair value		1,247,327	1,454,608
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		516	19,591
Gross unrealized gains on Fixed maturities		111	2,789
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities, at fair value		627	22,380
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		96,055	77,762
Gross unrealized gains on Fixed maturities		25,039	16,478
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities, at fair value		121,094	94,240
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		156,072	84,541
Gross unrealized gains on Fixed maturities		37,006	17,019
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities, at fair value		193,078	101,560
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.