Investments - Contractual maturities of income securities (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
AFS securities
Amortized cost of Fixed maturities	$ 31,517,654	$ 27,728,771
Fixed maturities, at fair value	32,257,589	29,359,034
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	1,263,609	1,180
Fixed maturities, trading, at fair value	1,235,699	1,171
Exclude Life Funds Withheld Assets [Member]
AFS securities
Due less than one year, amortized cost	2,106,851	1,972,224
Due less than one year, fair value	2,104,106	1,980,429
Due after 1 through 5 years, amortized cost	14,051,494	8,919,037
Due after 1 through 5 years, fair value	14,143,461	9,113,651
Due after 5 through 10 years, amortized cost	5,680,830	4,232,396
Due after 5 through 10 years, fair value	5,740,954	4,412,569
Due after 10 years, amortized cost	1,374,997	1,494,315
Due after 10 years, fair value	1,423,156	1,705,022
Subtotal - amortized cost	23,214,172	16,617,972
Subtotal - fair value	23,411,677	17,211,671
Mortgage and asset-backed securities - amortized cost	5,625,450	6,809,460
Mortgage and asset-backed securities - fair value	5,692,809	6,965,798
Amortized cost of Fixed maturities	28,839,622	23,427,432
Fixed maturities, at fair value	29,104,486	24,177,469
Life Funds Withheld Assets [Member]
AFS securities
Due less than one year, amortized cost	81,700	117,048
Due less than one year, fair value	92,921	125,326
Due after 1 through 5 years, amortized cost	386,810	638,526
Due after 1 through 5 years, fair value	416,743	685,787
Due after 5 through 10 years, amortized cost	491,621	1,004,698
Due after 5 through 10 years, fair value	558,805	1,165,348
Due after 10 years, amortized cost	1,452,267	2,059,637
Due after 10 years, fair value	1,784,653	2,649,279
Subtotal - amortized cost	2,412,398	3,819,909
Subtotal - fair value	2,853,122	4,625,740
Mortgage and asset-backed securities - amortized cost	265,634	481,430
Mortgage and asset-backed securities - fair value	299,981	555,825
Amortized cost of Fixed maturities	2,678,032	4,301,339
Fixed maturities, at fair value	3,153,103	5,181,565
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	1,263,609
Fixed maturities, trading, at fair value	1,235,699
Life Funds Withheld Assets [Member] | Cost [Member]
Trading Securities [Abstract]
Trading securities due less than one year	4,573	0
Trading securities due after 1 through 5 years	278,163	0
Trading securities due after 5 through 10 years	280,487	1,180
Trading securities due after 10 years	668,737	0
Trading Securities Debt Maturities And Short Term Investments Fixed Maturities	1,231,960	1,180
Trading Securities Debt Maturities Without Single Maturity Date	31,649	0
Amortized cost of fixed maturities, trading	1,263,609	1,180
Life Funds Withheld Assets [Member] | Fair value [Member]
Trading Securities [Abstract]
Trading securities due less than one year	5,096	0
Trading securities due after 1 through 5 years	272,220	0
Trading securities due after 5 through 10 years	277,920	1,171
Trading securities due after 10 years	649,592	0
Trading Securities Debt Maturities And Short Term Investments Fixed Maturities	1,204,828	1,171
Trading Securities Debt Maturities Without Single Maturity Date	30,871	0
Fixed maturities, trading, at fair value	1,235,699	1,171
RMBS - Agency | Exclude Life Funds Withheld Assets [Member]
AFS securities
Mortgage and asset-backed securities - amortized cost	3,697,756	3,625,171
Mortgage and asset-backed securities - fair value	3,757,971	3,728,576
Amortized cost of Fixed maturities	3,697,756	3,625,171
Fixed maturities, at fair value	3,757,971	3,728,576
RMBS - Agency | Life Funds Withheld Assets [Member]
AFS securities
Mortgage and asset-backed securities - amortized cost	591	3,301
Mortgage and asset-backed securities - fair value	752	3,782
Amortized cost of Fixed maturities	591	3,301
Fixed maturities, at fair value	752	3,782
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	370
Fixed maturities, trading, at fair value	368
RMBS - Agency | Life Funds Withheld Assets [Member] | Cost [Member]
Trading Securities [Abstract]
Trading Securities Debt Maturities Without Single Maturity Date	370	0
RMBS - Agency | Life Funds Withheld Assets [Member] | Fair value [Member]
Trading Securities [Abstract]
Trading Securities Debt Maturities Without Single Maturity Date	368	0
RMBS - Non-Agency | Exclude Life Funds Withheld Assets [Member]
AFS securities
Mortgage and asset-backed securities - amortized cost	319,876	404,398
Mortgage and asset-backed securities - fair value	328,540	427,351
Amortized cost of Fixed maturities	319,876	404,398
Fixed maturities, at fair value	328,540	427,351
RMBS - Non-Agency | Life Funds Withheld Assets [Member]
AFS securities
Mortgage and asset-backed securities - amortized cost	24,401	71,075
Mortgage and asset-backed securities - fair value	26,953	85,335
Amortized cost of Fixed maturities	24,401	71,075
Fixed maturities, at fair value	26,953	85,335
CMBS | Exclude Life Funds Withheld Assets [Member]
AFS securities
Mortgage and asset-backed securities - amortized cost	401,713	1,033,819
Mortgage and asset-backed securities - fair value	405,316	1,052,544
Amortized cost of Fixed maturities	401,713	1,033,819
Fixed maturities, at fair value	405,316	1,052,544
CMBS | Life Funds Withheld Assets [Member]
AFS securities
Mortgage and asset-backed securities - amortized cost	107,968	168,886
Mortgage and asset-backed securities - fair value	122,481	193,167
Amortized cost of Fixed maturities	107,968	168,886
Fixed maturities, at fair value	122,481	193,167
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	4,874
Fixed maturities, trading, at fair value	4,803
CMBS | Life Funds Withheld Assets [Member] | Cost [Member]
Trading Securities [Abstract]
Trading Securities Debt Maturities Without Single Maturity Date	4,874	0
CMBS | Life Funds Withheld Assets [Member] | Fair value [Member]
Trading Securities [Abstract]
Trading Securities Debt Maturities Without Single Maturity Date	4,803	0
Collateralized debt obligations ('CDO') [Member] | Exclude Life Funds Withheld Assets [Member]
AFS securities
Mortgage and asset-backed securities - amortized cost	41,679	717,544
Mortgage and asset-backed securities - fair value	32,410	692,034
Amortized cost of Fixed maturities	41,679	717,544
Fixed maturities, at fair value	32,410	692,034
Other asset-backed securities | Exclude Life Funds Withheld Assets [Member]
AFS securities
Mortgage and asset-backed securities - amortized cost	1,164,426	1,028,528
Mortgage and asset-backed securities - fair value	1,168,572	1,065,293
Amortized cost of Fixed maturities	1,164,426	1,028,528
Fixed maturities, at fair value	1,168,572	1,065,293
Other asset-backed securities | Life Funds Withheld Assets [Member]
AFS securities
Mortgage and asset-backed securities - amortized cost	132,674	238,168
Mortgage and asset-backed securities - fair value	149,795	273,541
Amortized cost of Fixed maturities	132,674	238,168
Fixed maturities, at fair value	149,795	273,541
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	26,405
Fixed maturities, trading, at fair value	25,700
Other asset-backed securities | Life Funds Withheld Assets [Member] | Cost [Member]
Trading Securities [Abstract]
Trading Securities Debt Maturities Without Single Maturity Date	26,405	0
Other asset-backed securities | Life Funds Withheld Assets [Member] | Fair value [Member]
Trading Securities [Abstract]
Trading Securities Debt Maturities Without Single Maturity Date	$ 25,700	$ 0