Investments - Contractual maturities of available for sale securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fixed Maturities - AFS
Amortized cost of fixed maturities	$ 30,268,346	$ 30,157,581
Fixed maturities, at fair value	30,656,628	30,928,988
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	2,055,722	1,946,501
Fixed maturities, trading at fair value	2,084,251	2,006,385
Excluding Life Funds Withheld Assets
Fixed Maturities - AFS
Due less than one year, amortized cost	1,716,039	1,556,688
Due less than one year, fair value	1,717,129	1,564,360
Due after 1 through 5 years, amortized cost	12,520,942	12,243,590
Due after 1 through 5 years, fair value	12,501,887	12,309,732
Due after 5 through 10 years, amortized cost	5,882,207	6,268,217
Due after 5 through 10 years, fair value	5,876,041	6,362,314
Due after 10 years, amortized cost	1,061,776	1,048,129
Due after 10 years, fair value	1,155,385	1,187,688
Subtotal - amortized cost	21,180,964	21,116,624
Subtotal - fair value	21,250,442	21,424,094
Mortgage and asset backed securities - amortized cost	7,487,471	7,452,906
Mortgage and asset backed securities - fair value	7,397,638	7,477,782
Amortized cost of fixed maturities	28,668,435	28,569,530
Fixed maturities, at fair value	28,648,080	28,901,876
Excluding Life Funds Withheld Assets | Residential mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	4,606,275	4,717,542
Mortgage and asset backed securities - fair value	4,529,723	4,724,402
Amortized cost of fixed maturities	4,606,275	4,717,542
Fixed maturities, at fair value	4,529,723	4,724,402
Excluding Life Funds Withheld Assets | Commercial mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	1,292,559	1,246,406
Mortgage and asset backed securities - fair value	1,259,519	1,244,017
Amortized cost of fixed maturities	1,292,559	1,246,406
Fixed maturities, at fair value	1,259,519	1,244,017
Excluding Life Funds Withheld Assets | Other asset-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	1,588,637	1,488,958
Mortgage and asset backed securities - fair value	1,608,396	1,509,363
Amortized cost of fixed maturities	1,588,637	1,488,958
Fixed maturities, at fair value	1,608,396	1,509,363
Life Funds Withheld Assets
Fixed Maturities - AFS
Due less than one year, amortized cost	66,237	47,143
Due less than one year, fair value	68,315	49,233
Due after 1 through 5 years, amortized cost	274,247	286,524
Due after 1 through 5 years, fair value	296,715	313,227
Due after 5 through 10 years, amortized cost	167,949	168,897
Due after 5 through 10 years, fair value	201,133	205,536
Due after 10 years, amortized cost	852,528	849,739
Due after 10 years, fair value	1,151,062	1,165,067
Subtotal - amortized cost	1,360,961	1,352,303
Subtotal - fair value	1,717,225	1,733,063
Mortgage and asset backed securities - amortized cost	238,950	235,748
Mortgage and asset backed securities - fair value	291,323	294,049
Amortized cost of fixed maturities	1,599,911	1,588,051
Fixed maturities, at fair value	2,008,548	2,027,112
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	2,055,722	1,946,501
Fixed maturities, trading at fair value	2,084,251	2,006,385
Life Funds Withheld Assets | Residential mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	455	448
Mortgage and asset backed securities - fair value	503	515
Amortized cost of fixed maturities	455	448
Fixed maturities, at fair value	503	515
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	1,023	1,020
Fixed maturities, trading at fair value	893	932
Life Funds Withheld Assets | Commercial mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	105,505	97,356
Mortgage and asset backed securities - fair value	130,070	122,272
Amortized cost of fixed maturities	105,505	97,356
Fixed maturities, at fair value	130,070	122,272
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	7,358	7,345
Fixed maturities, trading at fair value	7,472	7,580
Life Funds Withheld Assets | Other asset-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	132,990	137,944
Mortgage and asset backed securities - fair value	160,750	171,262
Amortized cost of fixed maturities	132,990	137,944
Fixed maturities, at fair value	160,750	171,262
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	44,238	39,771
Fixed maturities, trading at fair value	43,697	41,337
Life Funds Withheld Assets | Cost
Trading Securities [Abstract]
Due less than one year	66,532	59,962
Due after 1 through 5 years	493,369	486,847
Due after 5 through 10 years	691,074	645,573
Due after 10 years	752,128	705,983
Subtotal	2,003,103	1,898,365
Mortgage and asset-backed securities	52,619	48,136
Amortized cost of fixed maturities, trading	2,055,722	1,946,501
Life Funds Withheld Assets | Cost | Residential mortgage-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	1,023	1,020
Life Funds Withheld Assets | Cost | Commercial mortgage-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	7,358	7,345
Life Funds Withheld Assets | Cost | Other asset-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	44,238	39,771
Life Funds Withheld Assets | Fair value
Trading Securities [Abstract]
Due less than one year	65,428	59,605
Due after 1 through 5 years	495,899	492,998
Due after 5 through 10 years	690,354	657,093
Due after 10 years	780,508	746,840
Subtotal	2,032,189	1,956,536
Mortgage and asset-backed securities	52,062	49,849
Fixed maturities, trading at fair value	2,084,251	2,006,385
Life Funds Withheld Assets | Fair value | Residential mortgage-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	893	932
Life Funds Withheld Assets | Fair value | Commercial mortgage-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	7,472	7,580
Life Funds Withheld Assets | Fair value | Other asset-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	$ 43,697	$ 41,337