Investments - Contractual maturities of income securities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
AFS securities
Amortized cost of Fixed maturities	$ 27,728,771	$ 27,111,874
Fixed maturities, at fair value	29,359,034	27,500,136
Fixed maturities - HTM
Due less than one year, amortized cost	0	65,651
Due less than one year, fair value	0	66,766
Due after 1 through 5 years, amortized cost	0	240,802
Due after 1 through 5 years, fair value	0	255,322
Due after 5 through 10 years, amortized cost	0	455,633
Due after 5 through 10 years, fair value	0	492,095
Due after 10 years, amortized cost	0	1,778,158
Due after 10 years, fair value	0	1,974,844
Subtotal - amortized cost	0	2,540,244
Subtotal - fair value	0	2,789,027
Mortage and asset-backed securities - amortized cost	0	318,451
Mortgage and asset-backed securities - fair value	0	342,208
Fixed maturities, held to maturity	0	2,858,695
Held to maturity, fair value	0	3,131,235
RMBS - Agency
AFS securities
Amortized cost of Fixed maturities		3,540,101
Fixed maturities, at fair value		3,546,122
RMBS - Non-Agency
AFS securities
Amortized cost of Fixed maturities		396,798
Fixed maturities, at fair value		398,768
Fixed maturities - HTM
Mortage and asset-backed securities - amortized cost	0	66,987
Mortgage and asset-backed securities - fair value	0	71,972
Fixed maturities, held to maturity		66,987
Held to maturity, fair value		71,972
CMBS
AFS securities
Amortized cost of Fixed maturities		1,223,313
Fixed maturities, at fair value		1,246,795
Fixed maturities - HTM
Mortage and asset-backed securities - amortized cost	0	144,924
Mortgage and asset-backed securities - fair value	0	156,788
Fixed maturities, held to maturity		144,924
Held to maturity, fair value		156,788
Collateralized debt obligations ('CDO') [Member]
AFS securities
Amortized cost of Fixed maturities		754,414
Fixed maturities, at fair value		717,313
Other asset-backed securities
AFS securities
Amortized cost of Fixed maturities		1,210,384
Fixed maturities, at fair value		1,242,104
Fixed maturities - HTM
Mortage and asset-backed securities - amortized cost	0	106,540
Mortgage and asset-backed securities - fair value	0	113,448
Fixed maturities, held to maturity		106,540
Held to maturity, fair value		113,448
Exclude Life Funds Withheld Assets [Member]
AFS securities
Due less than one year, amortized cost	1,972,224	2,052,251
Due less than one year, fair value	1,980,429	2,060,365
Due after 1 through 5 years, amortized cost	8,919,037	10,075,087
Due after 1 through 5 years, fair value	9,113,651	10,305,986
Due after 5 through 10 years, amortized cost	4,232,396	5,474,120
Due after 5 through 10 years, fair value	4,412,569	5,507,450
Due after 10 years, amortized cost	1,494,315	2,385,406
Due after 10 years, fair value	1,705,022	2,475,233
Subtotal - amortized cost	16,617,972	19,986,864
Subtotal - fair value	17,211,671	20,349,034
Mortgage and asset-backed securities - amortized cost	6,809,460	7,125,010
Mortgage and asset-backed securities - fair value	6,965,798	7,151,102
Amortized cost of Fixed maturities	23,427,432	27,111,874
Fixed maturities, at fair value	24,177,469	27,500,136
Exclude Life Funds Withheld Assets [Member] | RMBS - Agency
AFS securities
Mortgage and asset-backed securities - amortized cost	3,625,171	3,540,101
Mortgage and asset-backed securities - fair value	3,728,576	3,546,122
Amortized cost of Fixed maturities	3,625,171
Fixed maturities, at fair value	3,728,576
Exclude Life Funds Withheld Assets [Member] | RMBS - Non-Agency
AFS securities
Mortgage and asset-backed securities - amortized cost	404,398	396,798
Mortgage and asset-backed securities - fair value	427,351	398,768
Amortized cost of Fixed maturities	404,398
Fixed maturities, at fair value	427,351
Exclude Life Funds Withheld Assets [Member] | CMBS
AFS securities
Mortgage and asset-backed securities - amortized cost	1,033,819	1,223,313
Mortgage and asset-backed securities - fair value	1,052,544	1,246,795
Amortized cost of Fixed maturities	1,033,819
Fixed maturities, at fair value	1,052,544
Exclude Life Funds Withheld Assets [Member] | Collateralized debt obligations ('CDO') [Member]
AFS securities
Mortgage and asset-backed securities - amortized cost	717,544	754,414
Mortgage and asset-backed securities - fair value	692,034	717,313
Amortized cost of Fixed maturities	717,544
Fixed maturities, at fair value	692,034
Exclude Life Funds Withheld Assets [Member] | Other asset-backed securities
AFS securities
Mortgage and asset-backed securities - amortized cost	1,028,528	1,210,384
Mortgage and asset-backed securities - fair value	1,065,293	1,242,104
Amortized cost of Fixed maturities	1,028,528
Fixed maturities, at fair value	1,065,293
Life Funds Withheld Assets [Member]
AFS securities
Due less than one year, amortized cost	117,048	0
Due less than one year, fair value	125,326	0
Due after 1 through 5 years, amortized cost	638,526	0
Due after 1 through 5 years, fair value	685,787	0
Due after 5 through 10 years, amortized cost	1,004,698	0
Due after 5 through 10 years, fair value	1,165,348	0
Due after 10 years, amortized cost	2,059,637	0
Due after 10 years, fair value	2,649,279	0
Subtotal - amortized cost	3,819,909	0
Subtotal - fair value	4,625,740	0
Mortgage and asset-backed securities - amortized cost	481,430	0
Mortgage and asset-backed securities - fair value	555,825	0
Amortized cost of Fixed maturities	4,301,339	0
Fixed maturities, at fair value	5,181,565	0
Life Funds Withheld Assets [Member] | RMBS - Agency
AFS securities
Mortgage and asset-backed securities - amortized cost	3,301	0
Mortgage and asset-backed securities - fair value	3,782	0
Amortized cost of Fixed maturities	3,301
Fixed maturities, at fair value	3,782
Life Funds Withheld Assets [Member] | RMBS - Non-Agency
AFS securities
Mortgage and asset-backed securities - amortized cost	71,075	0
Mortgage and asset-backed securities - fair value	85,335	0
Amortized cost of Fixed maturities	71,075
Fixed maturities, at fair value	85,335
Life Funds Withheld Assets [Member] | CMBS
AFS securities
Mortgage and asset-backed securities - amortized cost	168,886	0
Mortgage and asset-backed securities - fair value	193,167	0
Amortized cost of Fixed maturities	168,886
Fixed maturities, at fair value	193,167
Life Funds Withheld Assets [Member] | Collateralized debt obligations ('CDO') [Member]
AFS securities
Mortgage and asset-backed securities - amortized cost	0	0
Mortgage and asset-backed securities - fair value	0	0
Amortized cost of Fixed maturities	0
Fixed maturities, at fair value	0
Life Funds Withheld Assets [Member] | Other asset-backed securities
AFS securities
Mortgage and asset-backed securities - amortized cost	238,168	0
Mortgage and asset-backed securities - fair value	273,541	0
Amortized cost of Fixed maturities	238,168
Fixed maturities, at fair value	273,541
Cost [Member] | Life Funds Withheld Assets [Member]
Trading Securities [Abstract]
Trading securities due after 1 through 5 years	0	0
Trading securities due after 5 through 10 years	1,180	0
Trading securities due after 10 years	0	0
Trading securities due less than one year	0	0
Total trading fixed maturities	1,180	0
Fair value [Member] | Life Funds Withheld Assets [Member]
Trading Securities [Abstract]
Trading securities due after 1 through 5 years	0	0
Trading securities due after 5 through 10 years	1,171	0
Trading securities due after 10 years	0	0
Trading securities due less than one year	0	0
Total trading fixed maturities	$ 1,171	$ 0