Investments - Contractual maturities of available for sale securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fixed Maturities - AFS
Amortized cost of fixed maturities	$ 29,689,322	$ 30,157,581
Fixed maturities, at fair value	29,793,151	30,928,988
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	1,965,372	1,946,501
Fixed maturities, trading at fair value	1,990,344	2,006,385
Excluding Life Funds Withheld Assets
Fixed Maturities - AFS
Due less than one year, amortized cost	1,763,966	1,556,688
Due less than one year, fair value	1,764,182	1,564,360
Due after 1 through 5 years, amortized cost	12,570,233	12,243,590
Due after 1 through 5 years, fair value	12,501,952	12,309,732
Due after 5 through 10 years, amortized cost	5,728,279	6,268,217
Due after 5 through 10 years, fair value	5,658,654	6,362,314
Due after 10 years, amortized cost	908,935	1,048,129
Due after 10 years, fair value	959,600	1,187,688
Subtotal - amortized cost	20,971,413	21,116,624
Subtotal - fair value	20,884,388	21,424,094
Mortgage and asset backed securities - amortized cost	7,441,928	7,452,906
Mortgage and asset backed securities - fair value	7,317,029	7,477,782
Amortized cost of fixed maturities	28,413,341	28,569,530
Fixed maturities, at fair value	28,201,417	28,901,876
Excluding Life Funds Withheld Assets | Residential mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	4,348,536	4,717,542
Mortgage and asset backed securities - fair value	4,254,036	4,724,402
Amortized cost of fixed maturities	4,348,536	4,717,542
Fixed maturities, at fair value	4,254,036	4,724,402
Excluding Life Funds Withheld Assets | Commercial mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	1,410,363	1,246,406
Mortgage and asset backed securities - fair value	1,363,850	1,244,017
Amortized cost of fixed maturities	1,410,363	1,246,406
Fixed maturities, at fair value	1,363,850	1,244,017
Excluding Life Funds Withheld Assets | Other asset-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	1,683,029	1,488,958
Mortgage and asset backed securities - fair value	1,699,143	1,509,363
Amortized cost of fixed maturities	1,683,029	1,488,958
Fixed maturities, at fair value	1,699,143	1,509,363
Life Funds Withheld Assets
Fixed Maturities - AFS
Due less than one year, amortized cost	47,308	47,143
Due less than one year, fair value	48,710	49,233
Due after 1 through 5 years, amortized cost	218,872	286,524
Due after 1 through 5 years, fair value	235,681	313,227
Due after 5 through 10 years, amortized cost	146,695	168,897
Due after 5 through 10 years, fair value	174,746	205,536
Due after 10 years, amortized cost	662,866	849,739
Due after 10 years, fair value	889,311	1,165,067
Subtotal - amortized cost	1,075,741	1,352,303
Subtotal - fair value	1,348,448	1,733,063
Mortgage and asset backed securities - amortized cost	200,240	235,748
Mortgage and asset backed securities - fair value	243,286	294,049
Amortized cost of fixed maturities	1,275,981	1,588,051
Fixed maturities, at fair value	1,591,734	2,027,112
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	1,965,372	1,946,501
Fixed maturities, trading at fair value	1,990,344	2,006,385
Life Funds Withheld Assets | Residential mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	412	448
Mortgage and asset backed securities - fair value	483	515
Amortized cost of fixed maturities	412	448
Fixed maturities, at fair value	483	515
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	985	1,020
Fixed maturities, trading at fair value	919	932
Life Funds Withheld Assets | Commercial mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	97,003	97,356
Mortgage and asset backed securities - fair value	119,611	122,272
Amortized cost of fixed maturities	97,003	97,356
Fixed maturities, at fair value	119,611	122,272
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	7,227	7,345
Fixed maturities, trading at fair value	7,411	7,580
Life Funds Withheld Assets | Other asset-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	102,825	137,944
Mortgage and asset backed securities - fair value	123,192	171,262
Amortized cost of fixed maturities	102,825	137,944
Fixed maturities, at fair value	123,192	171,262
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	42,762	39,771
Fixed maturities, trading at fair value	42,533	41,337
Life Funds Withheld Assets | Cost
Trading Securities [Abstract]
Due less than one year	48,626	59,962
Due after 1 through 5 years	519,812	486,847
Due after 5 through 10 years	664,273	645,573
Due after 10 years	681,687	705,983
Subtotal	1,914,398	1,898,365
Mortgage and asset-backed securities	50,974	48,136
Amortized cost of fixed maturities, trading	1,965,372	1,946,501
Life Funds Withheld Assets | Cost | Residential mortgage-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	985	1,020
Life Funds Withheld Assets | Cost | Commercial mortgage-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	7,227	7,345
Life Funds Withheld Assets | Cost | Other asset-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	42,762	39,771
Life Funds Withheld Assets | Fair value
Trading Securities [Abstract]
Due less than one year	47,686	59,605
Due after 1 through 5 years	522,579	492,998
Due after 5 through 10 years	665,029	657,093
Due after 10 years	704,187	746,840
Subtotal	1,939,481	1,956,536
Mortgage and asset-backed securities	50,863	49,849
Fixed maturities, trading at fair value	1,990,344	2,006,385
Life Funds Withheld Assets | Fair value | Residential mortgage-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	919	932
Life Funds Withheld Assets | Fair value | Commercial mortgage-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	7,411	7,580
Life Funds Withheld Assets | Fair value | Other asset-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	$ 42,533	$ 41,337