Investments - Contractual maturities of available for sale securities (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Fixed Maturities - AFS
Amortized cost of fixed maturities	$ 30,402,019	$ 31,517,654
Fixed maturities, at fair value	31,934,135	32,257,589
Amortized cost of fixed maturities, trading	1,480,434	1,263,609
Fixed maturities, trading at fair value	1,608,882	1,235,699
Excluding Life Funds Withheld Assets
Fixed Maturities - AFS
Due less than one year, amortized cost	2,132,413	2,106,851
Due less than one year, fair value	2,137,838	2,104,106
Due after 1 through 5 years, amortized cost	12,900,781	14,051,494
Due after 1 through 5 years, fair value	13,142,832	14,143,461
Due after 5 through 10 years, amortized cost	5,663,250	5,680,830
Due after 5 through 10 years, fair value	5,927,235	5,740,954
Due after 10 years, amortized cost	1,463,631	1,374,997
Due after 10 years, fair value	1,643,662	1,423,156
Subtotal - amortized cost	22,160,075	23,214,172
Subtotal - fair value	22,851,567	23,411,677
Mortgage and asset backed securities - amortized cost	6,140,857	5,625,450
Mortgage and asset backed securities - fair value	6,288,256	5,692,809
Amortized cost of fixed maturities	28,300,932	28,839,622
Fixed maturities, at fair value	29,139,823	29,104,486
Excluding Life Funds Withheld Assets | RMBS - Agency
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	4,469,725	3,697,756
Mortgage and asset backed securities - fair value	4,591,774	3,757,971
Amortized cost of fixed maturities	4,469,725	3,697,756
Fixed maturities, at fair value	4,591,774	3,757,971
Excluding Life Funds Withheld Assets | RMBS - Non-Agency
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	279,130	319,876
Mortgage and asset backed securities - fair value	285,927	328,540
Amortized cost of fixed maturities	279,130	319,876
Fixed maturities, at fair value	285,927	328,540
Excluding Life Funds Withheld Assets | Commercial mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	288,544	401,713
Mortgage and asset backed securities - fair value	296,104	405,316
Amortized cost of fixed maturities	288,544	401,713
Fixed maturities, at fair value	296,104	405,316
Excluding Life Funds Withheld Assets | Collateralized debt obligations
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	143,783	41,679
Mortgage and asset backed securities - fair value	141,281	32,410
Amortized cost of fixed maturities	143,783	41,679
Fixed maturities, at fair value	141,281	32,410
Excluding Life Funds Withheld Assets | Other asset-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	959,675	1,164,426
Mortgage and asset backed securities - fair value	973,170	1,168,572
Amortized cost of fixed maturities	959,675	1,164,426
Fixed maturities, at fair value	973,170	1,168,572
Life Funds Withheld Assets
Fixed Maturities - AFS
Due less than one year, amortized cost	70,372	81,700
Due less than one year, fair value	82,442	92,921
Due after 1 through 5 years, amortized cost	316,288	386,810
Due after 1 through 5 years, fair value	348,173	416,743
Due after 5 through 10 years, amortized cost	369,152	491,621
Due after 5 through 10 years, fair value	453,723	558,805
Due after 10 years, amortized cost	1,148,415	1,452,267
Due after 10 years, fair value	1,665,340	1,784,653
Subtotal - amortized cost	1,904,227	2,412,398
Subtotal - fair value	2,549,678	2,853,122
Mortgage and asset backed securities - amortized cost	196,860	265,634
Mortgage and asset backed securities - fair value	244,634	299,981
Amortized cost of fixed maturities	2,101,087	2,678,032
Fixed maturities, at fair value	2,794,312	3,153,103
Amortized cost of fixed maturities, trading	1,480,434	1,263,609
Fixed maturities, trading at fair value	1,608,882	1,235,699
Life Funds Withheld Assets | RMBS - Agency
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	537	591
Mortgage and asset backed securities - fair value	671	752
Amortized cost of fixed maturities	537	591
Fixed maturities, at fair value	671	752
Amortized cost of fixed maturities, trading	1,674	370
Fixed maturities, trading at fair value	1,680	368
Life Funds Withheld Assets | RMBS - Non-Agency
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	49	24,401
Mortgage and asset backed securities - fair value	65	26,953
Amortized cost of fixed maturities	49	24,401
Fixed maturities, at fair value	65	26,953
Amortized cost of fixed maturities, trading	148
Fixed maturities, trading at fair value	151
Life Funds Withheld Assets | Commercial mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	83,828	107,968
Mortgage and asset backed securities - fair value	105,969	122,481
Amortized cost of fixed maturities	83,828	107,968
Fixed maturities, at fair value	105,969	122,481
Amortized cost of fixed maturities, trading	5,198	4,874
Fixed maturities, trading at fair value	5,614	4,803
Life Funds Withheld Assets | Other asset-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	112,446	132,674
Mortgage and asset backed securities - fair value	137,929	149,795
Amortized cost of fixed maturities	112,446	132,674
Fixed maturities, at fair value	137,929	149,795
Amortized cost of fixed maturities, trading	25,744	26,405
Fixed maturities, trading at fair value	27,699	25,700
Life Funds Withheld Assets | Cost
Fixed Maturities - AFS
Due less than one year	47,425	4,573
Due after 1 through 5 years	316,116	278,163
Due after 5 through 10 years	408,421	280,487
Due after 10 years	675,709	668,737
Subtotal	1,447,671	1,231,960
Mortgage and asset-backed securities	32,764	31,649
Amortized cost of fixed maturities, trading	1,480,435	1,263,609
Life Funds Withheld Assets | Cost | RMBS - Agency
Fixed Maturities - AFS
Mortgage and asset-backed securities	1,674	370
Life Funds Withheld Assets | Cost | RMBS - Non-Agency
Fixed Maturities - AFS
Mortgage and asset-backed securities	148	0
Life Funds Withheld Assets | Cost | Commercial mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset-backed securities	5,198	4,874
Life Funds Withheld Assets | Cost | Other asset-backed securities
Fixed Maturities - AFS
Mortgage and asset-backed securities	25,744	26,405
Life Funds Withheld Assets | Fair value
Fixed Maturities - AFS
Due less than one year	48,617	5,096
Due after 1 through 5 years	321,982	272,220
Due after 5 through 10 years	429,326	277,920
Due after 10 years	773,813	649,592
Subtotal	1,573,738	1,204,828
Mortgage and asset-backed securities	35,144	30,871
Fixed maturities, trading at fair value	1,608,882	1,235,699
Life Funds Withheld Assets | Fair value | RMBS - Agency
Fixed Maturities - AFS
Mortgage and asset-backed securities	1,680	368
Life Funds Withheld Assets | Fair value | RMBS - Non-Agency
Fixed Maturities - AFS
Mortgage and asset-backed securities	151	0
Life Funds Withheld Assets | Fair value | Commercial mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset-backed securities	5,614	4,803
Life Funds Withheld Assets | Fair value | Other asset-backed securities
Fixed Maturities - AFS
Mortgage and asset-backed securities	$ 27,699	$ 25,700