Investments - Contractual maturities of available for sale securities (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Fixed Maturities - AFS
Amortized cost of fixed maturities	$ 30,389,600	$ 31,517,654
Fixed maturities, at fair value	31,858,724	32,257,589
Amortized cost of fixed maturities, trading	1,385,588	1,263,609
Fixed maturities, trading at fair value	1,478,407	1,235,699
Excluding Life Funds Withheld Assets
Fixed Maturities - AFS
Due less than one year, amortized cost	1,971,202	2,106,851
Due less than one year, fair value	1,960,971	2,104,106
Due after 1 through 5 years, amortized cost	12,992,741	14,051,494
Due after 1 through 5 years, fair value	13,266,292	14,143,461
Due after 5 through 10 years, amortized cost	5,733,195	5,680,830
Due after 5 through 10 years, fair value	6,008,825	5,740,954
Due after 10 years, amortized cost	1,521,594	1,374,997
Due after 10 years, fair value	1,694,793	1,423,156
Subtotal - amortized cost	22,218,732	23,214,172
Subtotal - fair value	22,930,881	23,411,677
Mortgage and asset backed securities - amortized cost	5,940,111	5,625,450
Mortgage and asset backed securities - fair value	6,084,037	5,692,809
Amortized cost of fixed maturities	28,158,843	28,839,622
Fixed maturities, at fair value	29,014,918	29,104,486
Excluding Life Funds Withheld Assets | RMBS - Agency
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	4,330,183	3,697,756
Mortgage and asset backed securities - fair value	4,455,630	3,757,971
Amortized cost of fixed maturities	4,330,183	3,697,756
Fixed maturities, at fair value	4,455,630	3,757,971
Excluding Life Funds Withheld Assets | RMBS - Non-Agency
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	287,736	319,876
Mortgage and asset backed securities - fair value	288,815	328,540
Amortized cost of fixed maturities	287,736	319,876
Fixed maturities, at fair value	288,815	328,540
Excluding Life Funds Withheld Assets | Commercial mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	270,546	401,713
Mortgage and asset backed securities - fair value	278,314	405,316
Amortized cost of fixed maturities	270,546	401,713
Fixed maturities, at fair value	278,314	405,316
Excluding Life Funds Withheld Assets | Collateralized debt obligations
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	21,437	41,679
Mortgage and asset backed securities - fair value	17,800	32,410
Amortized cost of fixed maturities	21,437	41,679
Fixed maturities, at fair value	17,800	32,410
Excluding Life Funds Withheld Assets | Other asset-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	1,030,209	1,164,426
Mortgage and asset backed securities - fair value	1,043,478	1,168,572
Amortized cost of fixed maturities	1,030,209	1,164,426
Fixed maturities, at fair value	1,043,478	1,168,572
Life Funds Withheld Assets
Fixed Maturities - AFS
Due less than one year, amortized cost	87,483	81,700
Due less than one year, fair value	97,965	92,921
Due after 1 through 5 years, amortized cost	327,255	386,810
Due after 1 through 5 years, fair value	357,289	416,743
Due after 5 through 10 years, amortized cost	427,845	491,621
Due after 5 through 10 years, fair value	509,997	558,805
Due after 10 years, amortized cost	1,181,136	1,452,267
Due after 10 years, fair value	1,630,801	1,784,653
Subtotal - amortized cost	2,023,719	2,412,398
Subtotal - fair value	2,596,052	2,853,122
Mortgage and asset backed securities - amortized cost	207,038	265,634
Mortgage and asset backed securities - fair value	247,754	299,981
Amortized cost of fixed maturities	2,230,757	2,678,032
Fixed maturities, at fair value	2,843,806	3,153,103
Amortized cost of fixed maturities, trading	1,385,588	1,263,609
Fixed maturities, trading at fair value	1,478,407	1,235,699
Life Funds Withheld Assets | RMBS - Agency
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	545	591
Mortgage and asset backed securities - fair value	686	752
Amortized cost of fixed maturities	545	591
Fixed maturities, at fair value	686	752
Amortized cost of fixed maturities, trading	1,844	370
Fixed maturities, trading at fair value	1,868	368
Life Funds Withheld Assets | RMBS - Non-Agency
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	21,384	24,401
Mortgage and asset backed securities - fair value	24,252	26,953
Amortized cost of fixed maturities	21,384	24,401
Fixed maturities, at fair value	24,252	26,953
Amortized cost of fixed maturities, trading	190
Fixed maturities, trading at fair value	197
Life Funds Withheld Assets | Commercial mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	86,373	107,968
Mortgage and asset backed securities - fair value	105,019	122,481
Amortized cost of fixed maturities	86,373	107,968
Fixed maturities, at fair value	105,019	122,481
Amortized cost of fixed maturities, trading	4,970	4,874
Fixed maturities, trading at fair value	5,223	4,803
Life Funds Withheld Assets | Collateralized debt obligations
Fixed Maturities - AFS
Amortized cost of fixed maturities, trading	0
Fixed maturities, trading at fair value	0
Life Funds Withheld Assets | Other asset-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	98,736	132,674
Mortgage and asset backed securities - fair value	117,797	149,795
Amortized cost of fixed maturities	98,736	132,674
Fixed maturities, at fair value	117,797	149,795
Amortized cost of fixed maturities, trading	24,858	26,405
Fixed maturities, trading at fair value	25,965	25,700
Life Funds Withheld Assets | Cost
Fixed Maturities - AFS
Due less than one year	15,713	4,573
Due after 1 through 5 years	335,915	278,163
Due after 5 through 10 years	336,150	280,487
Due after 10 years	665,949	668,737
Subtotal	1,353,727	1,231,960
Mortgage and asset-backed securities	31,862	31,649
Amortized cost of fixed maturities, trading	1,385,589	1,263,609
Life Funds Withheld Assets | Cost | RMBS - Agency
Fixed Maturities - AFS
Mortgage and asset-backed securities	1,844	370
Life Funds Withheld Assets | Cost | RMBS - Non-Agency
Fixed Maturities - AFS
Mortgage and asset-backed securities	190	0
Life Funds Withheld Assets | Cost | Commercial mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset-backed securities	4,970	4,874
Life Funds Withheld Assets | Cost | Other asset-backed securities
Fixed Maturities - AFS
Mortgage and asset-backed securities	24,858	26,405
Life Funds Withheld Assets | Fair value
Fixed Maturities - AFS
Due less than one year	16,587	5,096
Due after 1 through 5 years	339,896	272,220
Due after 5 through 10 years	350,397	277,920
Due after 10 years	738,274	649,592
Subtotal	1,445,154	1,204,828
Mortgage and asset-backed securities	33,253	30,871
Fixed maturities, trading at fair value	1,478,407	1,235,699
Life Funds Withheld Assets | Fair value | RMBS - Agency
Fixed Maturities - AFS
Mortgage and asset-backed securities	1,868	368
Life Funds Withheld Assets | Fair value | RMBS - Non-Agency
Fixed Maturities - AFS
Mortgage and asset-backed securities	197	0
Life Funds Withheld Assets | Fair value | Commercial mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset-backed securities	5,223	4,803
Life Funds Withheld Assets | Fair value | Other asset-backed securities
Fixed Maturities - AFS
Mortgage and asset-backed securities	$ 25,965	$ 25,700