Investments - Contractual maturities of available for sale securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fixed Maturities - AFS
Amortized cost of fixed maturities	$ 30,822,936	$ 29,587,656
Fixed maturities, at fair value	31,675,509	30,256,602
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	1,823,406	1,548,178
Fixed maturities, trading at fair value	1,866,465	1,617,014
Excluding Life Funds Withheld Assets
Fixed Maturities - AFS
Due less than one year, amortized cost	1,686,672	2,121,088
Due less than one year, fair value	1,702,285	2,135,262
Due after 1 through 5 years, amortized cost	12,366,653	12,400,798
Due after 1 through 5 years, fair value	12,483,526	12,463,844
Due after 5 through 10 years, amortized cost	6,194,107	5,509,194
Due after 5 through 10 years, fair value	6,294,833	5,531,402
Due after 10 years, amortized cost	1,155,865	1,335,687
Due after 10 years, fair value	1,277,520	1,406,853
Subtotal - amortized cost	21,403,297	21,366,767
Subtotal - fair value	21,758,164	21,537,361
Mortgage and asset backed securities - amortized cost	7,731,928	6,402,162
Mortgage and asset backed securities - fair value	7,787,211	6,411,199
Amortized cost of fixed maturities	29,135,225	27,768,929
Fixed maturities, at fair value	29,545,375	27,948,560
Excluding Life Funds Withheld Assets | Residential mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	4,907,674	4,479,722
Mortgage and asset backed securities - fair value	4,941,702	4,492,625
Amortized cost of fixed maturities	4,907,674	4,479,722
Fixed maturities, at fair value	4,941,702	4,492,625
Excluding Life Funds Withheld Assets | Commercial mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	1,277,629	670,005
Mortgage and asset backed securities - fair value	1,277,994	665,186
Amortized cost of fixed maturities	1,277,629	670,005
Fixed maturities, at fair value	1,277,994	665,186
Excluding Life Funds Withheld Assets | Other asset-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	1,546,625	1,252,435
Mortgage and asset backed securities - fair value	1,567,515	1,253,388
Amortized cost of fixed maturities	1,546,625	1,252,435
Fixed maturities, at fair value	1,567,515	1,253,388
Life Funds Withheld Assets
Fixed Maturities - AFS
Due less than one year, amortized cost	31,584	71,207
Due less than one year, fair value	33,765	80,749
Due after 1 through 5 years, amortized cost	296,748	289,710
Due after 1 through 5 years, fair value	323,436	318,605
Due after 5 through 10 years, amortized cost	216,651	274,727
Due after 5 through 10 years, fair value	258,462	324,759
Due after 10 years, amortized cost	903,447	1,001,189
Due after 10 years, fair value	1,218,821	1,365,749
Subtotal - amortized cost	1,448,430	1,636,833
Subtotal - fair value	1,834,484	2,089,862
Mortgage and asset backed securities - amortized cost	239,281	181,894
Mortgage and asset backed securities - fair value	295,650	218,180
Amortized cost of fixed maturities	1,687,711	1,818,727
Fixed maturities, at fair value	2,130,134	2,308,042
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	1,823,406	1,548,178
Fixed maturities, trading at fair value	1,866,465	1,617,014
Life Funds Withheld Assets | Residential mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	503	19,591
Mortgage and asset backed securities - fair value	589	22,380
Amortized cost of fixed maturities	503	19,591
Fixed maturities, at fair value	589	22,380
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	886	936
Fixed maturities, trading at fair value	818	961
Life Funds Withheld Assets | Commercial mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	98,779	77,762
Mortgage and asset backed securities - fair value	122,223	94,240
Amortized cost of fixed maturities	98,779	77,762
Fixed maturities, at fair value	122,223	94,240
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	7,453	5,242
Fixed maturities, trading at fair value	7,581	5,526
Life Funds Withheld Assets | Other asset-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	139,999	84,541
Mortgage and asset backed securities - fair value	172,838	101,560
Amortized cost of fixed maturities	139,999	84,541
Fixed maturities, at fair value	172,838	101,560
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	40,932	38,342
Fixed maturities, trading at fair value	42,206	39,446
Life Funds Withheld Assets | Cost
Trading Securities [Abstract]
Due less than one year	57,208	59,138
Due after 1 through 5 years	481,836	322,574
Due after 5 through 10 years	577,671	507,631
Due after 10 years	657,420	614,315
Subtotal	1,774,135	1,503,658
Mortgage and asset-backed securities	49,271	44,520
Amortized cost of fixed maturities, trading	1,823,406	1,548,178
Life Funds Withheld Assets | Cost | Residential mortgage-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	886	936
Life Funds Withheld Assets | Cost | Commercial mortgage-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	7,453	5,242
Life Funds Withheld Assets | Cost | Other asset-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	40,932	38,342
Life Funds Withheld Assets | Fair value
Trading Securities [Abstract]
Due less than one year	56,657	59,553
Due after 1 through 5 years	487,343	328,997
Due after 5 through 10 years	585,182	521,505
Due after 10 years	686,678	661,026
Subtotal	1,815,860	1,571,081
Mortgage and asset-backed securities	50,605	45,933
Fixed maturities, trading at fair value	1,866,465	1,617,014
Life Funds Withheld Assets | Fair value | Residential mortgage-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	818	961
Life Funds Withheld Assets | Fair value | Commercial mortgage-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	7,581	5,526
Life Funds Withheld Assets | Fair value | Other asset-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	$ 42,206	$ 39,446