Investments - Contractual maturities of available for sale securities (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Fixed Maturities - AFS
Amortized cost of fixed maturities	$ 30,012,097	$ 29,587,656
Fixed maturities, at fair value	30,810,724	30,256,602
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	1,724,250	1,548,178
Fixed maturities, trading at fair value	1,772,877	1,617,014
Excluding Life Funds Withheld Assets
Fixed Maturities - AFS
Due less than one year, amortized cost	1,605,068	2,121,088
Due less than one year, fair value	1,621,615	2,135,262
Due after 1 through 5 years, amortized cost	12,991,237	12,400,798
Due after 1 through 5 years, fair value	13,087,897	12,463,844
Due after 5 through 10 years, amortized cost	6,178,863	5,509,194
Due after 5 through 10 years, fair value	6,262,475	5,531,402
Due after 10 years, amortized cost	1,222,607	1,335,687
Due after 10 years, fair value	1,333,125	1,406,853
Subtotal - amortized cost	21,997,775	21,366,767
Subtotal - fair value	22,305,112	21,537,361
Mortgage and asset backed securities - amortized cost	6,302,560	6,402,162
Mortgage and asset backed securities - fair value	6,336,827	6,411,199
Amortized cost of fixed maturities	28,300,335	27,768,929
Fixed maturities, at fair value	28,641,939	27,948,560
Excluding Life Funds Withheld Assets | Residential mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	3,937,837	4,479,722
Mortgage and asset backed securities - fair value	3,954,318	4,492,625
Amortized cost of fixed maturities	3,937,837	4,479,722
Fixed maturities, at fair value	3,954,318	4,492,625
Excluding Life Funds Withheld Assets | Commercial mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	794,364	670,005
Mortgage and asset backed securities - fair value	795,453	665,186
Amortized cost of fixed maturities	794,364	670,005
Fixed maturities, at fair value	795,453	665,186
Excluding Life Funds Withheld Assets | Other asset-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	1,570,359	1,252,435
Mortgage and asset backed securities - fair value	1,587,056	1,253,388
Amortized cost of fixed maturities	1,570,359	1,252,435
Fixed maturities, at fair value	1,587,056	1,253,388
Life Funds Withheld Assets
Fixed Maturities - AFS
Due less than one year, amortized cost	38,157	71,207
Due less than one year, fair value	40,665	80,749
Due after 1 through 5 years, amortized cost	269,121	289,710
Due after 1 through 5 years, fair value	294,782	318,605
Due after 5 through 10 years, amortized cost	241,742	274,727
Due after 5 through 10 years, fair value	287,171	324,759
Due after 10 years, amortized cost	910,099	1,001,189
Due after 10 years, fair value	1,231,368	1,365,749
Subtotal - amortized cost	1,459,119	1,636,833
Subtotal - fair value	1,853,986	2,089,862
Mortgage and asset backed securities - amortized cost	252,643	181,894
Mortgage and asset backed securities - fair value	314,799	218,180
Amortized cost of fixed maturities	1,711,762	1,818,727
Fixed maturities, at fair value	2,168,785	2,308,042
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	1,724,250	1,548,178
Fixed maturities, trading at fair value	1,772,877	1,617,014
Life Funds Withheld Assets | Residential mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	516	19,591
Mortgage and asset backed securities - fair value	627	22,380
Amortized cost of fixed maturities	516	19,591
Fixed maturities, at fair value	627	22,380
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	880	936
Fixed maturities, trading at fair value	838	961
Life Funds Withheld Assets | Commercial mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	96,055	77,762
Mortgage and asset backed securities - fair value	121,094	94,240
Amortized cost of fixed maturities	96,055	77,762
Fixed maturities, at fair value	121,094	94,240
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	7,101	5,242
Fixed maturities, trading at fair value	7,318	5,526
Life Funds Withheld Assets | Other asset-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	156,072	84,541
Mortgage and asset backed securities - fair value	193,078	101,560
Amortized cost of fixed maturities	156,072	84,541
Fixed maturities, at fair value	193,078	101,560
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	40,276	38,342
Fixed maturities, trading at fair value	41,633	39,446
Life Funds Withheld Assets | Cost
Trading Securities [Abstract]
Due less than one year	60,164	59,138
Due after 1 through 5 years	428,723	322,574
Due after 5 through 10 years	569,290	507,631
Due after 10 years	617,816	614,315
Subtotal	1,675,993	1,503,658
Mortgage and asset-backed securities	48,257	44,520
Amortized cost of fixed maturities, trading	1,724,250	1,548,178
Life Funds Withheld Assets | Cost | Residential mortgage-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	880	936
Life Funds Withheld Assets | Cost | Commercial mortgage-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	7,101	5,242
Life Funds Withheld Assets | Cost | Other asset-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	40,276	38,342
Life Funds Withheld Assets | Fair value
Trading Securities [Abstract]
Due less than one year	60,176	59,553
Due after 1 through 5 years	435,199	328,997
Due after 5 through 10 years	578,479	521,505
Due after 10 years	649,234	661,026
Subtotal	1,723,088	1,571,081
Mortgage and asset-backed securities	49,789	45,933
Fixed maturities, trading at fair value	1,772,877	1,617,014
Life Funds Withheld Assets | Fair value | Residential mortgage-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	838	961
Life Funds Withheld Assets | Fair value | Commercial mortgage-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	7,318	5,526
Life Funds Withheld Assets | Fair value | Other asset-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	$ 41,633	$ 39,446