Investments - Contractual maturities of income securities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	$ 1,548,178	$ 1,263,609
Fixed maturities, trading, at fair value	1,617,014	1,235,699
AFS securities
Amortized cost of Fixed maturities	29,587,656	31,517,654
Fixed maturities, at fair value	30,256,602	32,257,589
Exclude Life Funds Withheld Assets
AFS securities
Due less than one year, amortized cost	2,121,088	2,106,851
Due less than one year, fair value	2,135,262	2,104,106
Due after 1 through 5 years, amortized cost	12,400,798	14,051,494
Due after 1 through 5 years, fair value	12,463,844	14,143,461
Due after 5 through 10 years, amortized cost	5,509,194	5,680,830
Due after 5 through 10 years, fair value	5,531,402	5,740,954
Due after 10 years, amortized cost	1,335,687	1,374,997
Due after 10 years, fair value	1,406,853	1,423,156
Subtotal - amortized cost	21,366,767	23,214,172
Subtotal - fair value	21,537,361	23,411,677
Mortgage and asset-backed securities - amortized cost	6,402,162	5,625,450
Mortgage and asset-backed securities - fair value	6,411,199	5,692,809
Amortized cost of Fixed maturities	27,768,929	28,839,622
Fixed maturities, at fair value	27,948,560	29,104,486
Exclude Life Funds Withheld Assets | RMBS - Agency
AFS securities
Mortgage and asset-backed securities - amortized cost	4,213,845	3,697,756
Mortgage and asset-backed securities - fair value	4,218,367	3,757,971
Amortized cost of Fixed maturities	4,213,845	3,697,756
Fixed maturities, at fair value	4,218,367	3,757,971
Exclude Life Funds Withheld Assets | RMBS - Non-Agency
AFS securities
Mortgage and asset-backed securities - amortized cost	265,877	319,876
Mortgage and asset-backed securities - fair value	274,258	328,540
Amortized cost of Fixed maturities	265,877	319,876
Fixed maturities, at fair value	274,258	328,540
Exclude Life Funds Withheld Assets | CMBS
AFS securities
Mortgage and asset-backed securities - amortized cost	670,005	401,713
Mortgage and asset-backed securities - fair value	665,186	405,316
Amortized cost of Fixed maturities	670,005	401,713
Fixed maturities, at fair value	665,186	405,316
Exclude Life Funds Withheld Assets | Collateralized debt obligations ('CDO')
AFS securities
Mortgage and asset-backed securities - amortized cost	379,532	41,679
Mortgage and asset-backed securities - fair value	377,491	32,410
Amortized cost of Fixed maturities	379,532	41,679
Fixed maturities, at fair value	377,491	32,410
Exclude Life Funds Withheld Assets | Other asset-backed securities
AFS securities
Mortgage and asset-backed securities - amortized cost	872,903	1,164,426
Mortgage and asset-backed securities - fair value	875,897	1,168,572
Amortized cost of Fixed maturities	872,903	1,164,426
Fixed maturities, at fair value	875,897	1,168,572
Life Funds Withheld Assets
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	1,548,178	1,263,609
Fixed maturities, trading, at fair value	1,617,014	1,235,699
AFS securities
Due less than one year, amortized cost	71,207	81,700
Due less than one year, fair value	80,749	92,921
Due after 1 through 5 years, amortized cost	289,710	386,810
Due after 1 through 5 years, fair value	318,605	416,743
Due after 5 through 10 years, amortized cost	274,727	491,621
Due after 5 through 10 years, fair value	324,759	558,805
Due after 10 years, amortized cost	1,001,189	1,452,267
Due after 10 years, fair value	1,365,749	1,784,653
Subtotal - amortized cost	1,636,833	2,412,398
Subtotal - fair value	2,089,862	2,853,122
Mortgage and asset-backed securities - amortized cost	181,894	265,634
Mortgage and asset-backed securities - fair value	218,180	299,981
Amortized cost of Fixed maturities	1,818,727	2,678,032
Fixed maturities, at fair value	2,308,042	3,153,103
Life Funds Withheld Assets | RMBS - Agency
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	835	370
Fixed maturities, trading, at fair value	852	368
AFS securities
Mortgage and asset-backed securities - amortized cost	470	591
Mortgage and asset-backed securities - fair value	613	752
Amortized cost of Fixed maturities	470	591
Fixed maturities, at fair value	613	752
Life Funds Withheld Assets | RMBS - Non-Agency
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	101
Fixed maturities, trading, at fair value	109
AFS securities
Mortgage and asset-backed securities - amortized cost	19,121	24,401
Mortgage and asset-backed securities - fair value	21,767	26,953
Amortized cost of Fixed maturities	19,121	24,401
Fixed maturities, at fair value	21,767	26,953
Life Funds Withheld Assets | CMBS
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	5,242	4,874
Fixed maturities, trading, at fair value	5,526	4,803
AFS securities
Mortgage and asset-backed securities - amortized cost	77,762	107,968
Mortgage and asset-backed securities - fair value	94,240	122,481
Amortized cost of Fixed maturities	77,762	107,968
Fixed maturities, at fair value	94,240	122,481
Life Funds Withheld Assets | Other asset-backed securities
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	38,342	26,405
Fixed maturities, trading, at fair value	39,446	25,700
AFS securities
Mortgage and asset-backed securities - amortized cost	84,541	132,674
Mortgage and asset-backed securities - fair value	101,560	149,795
Amortized cost of Fixed maturities	84,541	132,674
Fixed maturities, at fair value	101,560	149,795
Cost [Member] | Life Funds Withheld Assets
Trading Securities [Abstract]
Trading securities due less than one year	59,138	4,573
Trading securities due after 1 through 5 years	322,574	278,163
Trading securities due after 5 through 10 years	507,631	280,487
Trading securities due after 10 years	614,315	668,737
Trading Securities Debt Maturities And Short Term Investments Fixed Maturities	1,503,658	1,231,960
Trading Securities Debt Maturities Without Single Maturity Date	44,520	31,649
Amortized cost of fixed maturities, trading	1,548,178	1,263,609
Cost [Member] | Life Funds Withheld Assets | RMBS - Agency
Trading Securities [Abstract]
Trading Securities Debt Maturities Without Single Maturity Date	835	370
Cost [Member] | Life Funds Withheld Assets | RMBS - Non-Agency
Trading Securities [Abstract]
Trading Securities Debt Maturities Without Single Maturity Date	101	0
Cost [Member] | Life Funds Withheld Assets | CMBS
Trading Securities [Abstract]
Trading Securities Debt Maturities Without Single Maturity Date	5,242	4,874
Cost [Member] | Life Funds Withheld Assets | Other asset-backed securities
Trading Securities [Abstract]
Trading Securities Debt Maturities Without Single Maturity Date	38,342	26,405
Fair value [Member] | Life Funds Withheld Assets
Trading Securities [Abstract]
Trading securities due less than one year	59,553	5,096
Trading securities due after 1 through 5 years	328,997	272,220
Trading securities due after 5 through 10 years	521,505	277,920
Trading securities due after 10 years	661,026	649,592
Trading Securities Debt Maturities And Short Term Investments Fixed Maturities	1,571,081	1,204,828
Trading Securities Debt Maturities Without Single Maturity Date	45,933	30,871
Fixed maturities, trading, at fair value	1,617,014	1,235,699
Fair value [Member] | Life Funds Withheld Assets | RMBS - Agency
Trading Securities [Abstract]
Trading Securities Debt Maturities Without Single Maturity Date	852	368
Fair value [Member] | Life Funds Withheld Assets | RMBS - Non-Agency
Trading Securities [Abstract]
Trading Securities Debt Maturities Without Single Maturity Date	109	0
Fair value [Member] | Life Funds Withheld Assets | CMBS
Trading Securities [Abstract]
Trading Securities Debt Maturities Without Single Maturity Date	5,526	4,803
Fair value [Member] | Life Funds Withheld Assets | Other asset-backed securities
Trading Securities [Abstract]
Trading Securities Debt Maturities Without Single Maturity Date	$ 39,446	$ 25,700