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		Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		$ 30,822,936	$ 29,587,656
Fixed maturities, at fair value		31,675,509	30,256,602
Non-credit Related OTTI	[1]	(46,422)	(51,135)
Amortized cost of short-term investments, available for sale		336,071	626,142
Short-term investments, at fair value		336,054	625,193
Cost of equity securities		920,130	996,610
Equity securities, at fair value		1,011,942	1,037,331
Available-for-sale Securities, Amortized Cost Basis		32,079,137	31,210,408
Available for sale securities, Gross unrealized gains		1,121,641	1,078,735
Available for sale securities, Gross unrealized losses		(177,273)	(370,017)
Total investments available for sale		33,023,505	31,919,126
Excluding Life Funds Withheld Assets
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		29,135,225	27,768,929
Gross unrealized gains on Fixed maturities		584,072	540,262
Gross unrealized losses on Fixed maturities		(173,922)	(360,631)
Fixed maturities, at fair value		29,545,375	27,948,560
Non-credit Related OTTI	[1]	(46,422)	(51,135)
Amortized cost of short-term investments, available for sale		336,071	626,142
Short term investments, gross unrealized gains		657	227
Short term investments, gross unrealized losses		(674)	(1,176)
Short-term investments, at fair value		336,054	625,193
Cost of equity securities		920,130	996,610
Equity securities, gross unrealized gains		94,489	48,931
Equity securities, gross unrealized losses		(2,677)	(8,210)
Equity securities, at fair value		1,011,942	1,037,331
Available-for-sale Securities, Amortized Cost Basis		30,391,426	29,391,681
Available for sale securities, Gross unrealized gains		679,218	589,420
Available for sale securities, Gross unrealized losses		(177,273)	(370,017)
Total investments available for sale		30,893,371	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		4,291,972	3,874,038
Gross unrealized gains on Fixed maturities		41,348	53,172
Gross unrealized losses on Fixed maturities		(19,984)	(32,822)
Fixed maturities, at fair value		4,313,336	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		2,009,472	2,399,490
Gross unrealized gains on Fixed maturities		102,610	86,041
Gross unrealized losses on Fixed maturities		(865)	(7,419)
Fixed maturities, at fair value		2,111,217	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		5,204,769	5,037,482
Gross unrealized gains on Fixed maturities		117,205	112,772
Gross unrealized losses on Fixed maturities		(61,337)	(120,122)
Fixed maturities, at fair value		5,260,637	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		9,897,084	10,055,757
Gross unrealized gains on Fixed maturities		229,162	198,082
Gross unrealized losses on Fixed maturities		(53,272)	(119,110)
Fixed maturities, at fair value		10,072,974	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		4,907,674	4,479,722
Gross unrealized gains on Fixed maturities		56,400	71,045
Gross unrealized losses on Fixed maturities		(22,372)	(58,142)
Fixed maturities, at fair value		4,941,702	4,492,625
Non-credit Related OTTI	[1]	(10,092)	(47,879)
Excluding Life Funds Withheld Assets | Commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		1,277,629	670,005
Gross unrealized gains on Fixed maturities		9,407	5,955
Gross unrealized losses on Fixed maturities		(9,042)	(10,774)
Fixed maturities, at fair value		1,277,994	665,186
Non-credit Related OTTI	[1]	(802)	(1,191)
Excluding Life Funds Withheld Assets | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		1,546,625	1,252,435
Gross unrealized gains on Fixed maturities		27,940	13,195
Gross unrealized losses on Fixed maturities		(7,050)	(12,242)
Fixed maturities, at fair value		1,567,515	1,253,388
Non-credit Related OTTI	[1]	(35,510)	(2,029)
Life Funds Withheld Assets
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of fixed maturities		1,687,711	1,818,727
Gross unrealized gains on Fixed maturities		442,423	489,315
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities, at fair value		2,130,134	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		9,020	8,468
Gross unrealized gains on Fixed maturities		2,070	2,109
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities, at fair value		11,090	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		457,936	454,695
Gross unrealized gains on Fixed maturities		153,907	169,982
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities, at fair value		611,843	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		981,474	1,173,670
Gross unrealized gains on Fixed maturities		230,077	280,938
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities, at fair value		1,211,551	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		503	19,591
Gross unrealized gains on Fixed maturities		86	2,789
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities, at fair value		589	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		98,779	77,762
Gross unrealized gains on Fixed maturities		23,444	16,478
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities, at fair value		122,223	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		139,999	84,541
Gross unrealized gains on Fixed maturities		32,839	17,019
Gross unrealized losses on Fixed maturities		0	0
Fixed maturities, at fair value		172,838	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.