Investments - Contractual maturities of available for sale securities (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Fixed Maturities - AFS
Amortized cost of fixed maturities	$ 29,923,912	$ 29,587,656
Fixed maturities, at fair value	30,651,790	30,256,602
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	1,631,045	1,548,178
Fixed maturities, trading at fair value	1,700,108	1,617,014
Excluding Life Funds Withheld Assets
Fixed Maturities - AFS
Due less than one year, amortized cost	2,033,484	2,121,088
Due less than one year, fair value	2,061,553	2,135,262
Due after 1 through 5 years, amortized cost	12,842,652	12,400,798
Due after 1 through 5 years, fair value	12,927,078	12,463,844
Due after 5 through 10 years, amortized cost	5,674,580	5,509,194
Due after 5 through 10 years, fair value	5,722,780	5,531,402
Due after 10 years, amortized cost	1,312,640	1,335,687
Due after 10 years, fair value	1,402,075	1,406,853
Subtotal - amortized cost	21,863,356	21,366,767
Subtotal - fair value	22,113,486	21,537,361
Mortgage and asset backed securities - amortized cost	6,319,595	6,402,162
Mortgage and asset backed securities - fair value	6,327,323	6,411,199
Amortized cost of fixed maturities	28,182,951	27,768,929
Fixed maturities, at fair value	28,440,809	27,948,560
Excluding Life Funds Withheld Assets | Residential mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	4,370,380	4,479,722
Mortgage and asset backed securities - fair value	4,377,435	4,492,625
Amortized cost of fixed maturities	4,370,380	4,479,722
Fixed maturities, at fair value	4,377,435	4,492,625
Excluding Life Funds Withheld Assets | Commercial mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	705,158	670,005
Mortgage and asset backed securities - fair value	700,197	665,186
Amortized cost of fixed maturities	705,158	670,005
Fixed maturities, at fair value	700,197	665,186
Excluding Life Funds Withheld Assets | Other asset-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	1,244,057	1,252,435
Mortgage and asset backed securities - fair value	1,249,691	1,253,388
Amortized cost of fixed maturities	1,244,057	1,252,435
Fixed maturities, at fair value	1,249,691	1,253,388
Life Funds Withheld Assets
Fixed Maturities - AFS
Due less than one year, amortized cost	58,836	71,207
Due less than one year, fair value	68,442	80,749
Due after 1 through 5 years, amortized cost	289,495	289,710
Due after 1 through 5 years, fair value	318,498	318,605
Due after 5 through 10 years, amortized cost	251,139	274,727
Due after 5 through 10 years, fair value	299,963	324,759
Due after 10 years, amortized cost	966,020	1,001,189
Due after 10 years, fair value	1,308,002	1,365,749
Subtotal - amortized cost	1,565,490	1,636,833
Subtotal - fair value	1,994,905	2,089,862
Mortgage and asset backed securities - amortized cost	175,471	181,894
Mortgage and asset backed securities - fair value	216,076	218,180
Amortized cost of fixed maturities	1,740,961	1,818,727
Fixed maturities, at fair value	2,210,981	2,308,042
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	1,631,045	1,548,178
Fixed maturities, trading at fair value	1,700,108	1,617,014
Life Funds Withheld Assets | Residential mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	19,267	19,591
Mortgage and asset backed securities - fair value	22,338	22,380
Amortized cost of fixed maturities	19,267	19,591
Fixed maturities, at fair value	22,338	22,380
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	919	936
Fixed maturities, trading at fair value	930	961
Life Funds Withheld Assets | Commercial mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	75,554	77,762
Mortgage and asset backed securities - fair value	93,810	94,240
Amortized cost of fixed maturities	75,554	77,762
Fixed maturities, at fair value	93,810	94,240
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	5,194	5,242
Fixed maturities, trading at fair value	5,550	5,526
Life Funds Withheld Assets | Other asset-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	80,650	84,541
Mortgage and asset backed securities - fair value	99,928	101,560
Amortized cost of fixed maturities	80,650	84,541
Fixed maturities, at fair value	99,928	101,560
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading	38,856	38,342
Fixed maturities, trading at fair value	40,484	39,446
Life Funds Withheld Assets | Cost
Trading Securities [Abstract]
Due less than one year	61,523	59,138
Due after 1 through 5 years	358,658	322,574
Due after 5 through 10 years	565,720	507,631
Due after 10 years	600,175	614,315
Subtotal	1,586,076	1,503,658
Mortgage and asset-backed securities	44,969	44,520
Amortized cost of fixed maturities, trading	1,631,045	1,548,178
Life Funds Withheld Assets | Cost | Residential mortgage-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	919	936
Life Funds Withheld Assets | Cost | Commercial mortgage-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	5,194	5,242
Life Funds Withheld Assets | Cost | Other asset-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	38,856	38,342
Life Funds Withheld Assets | Fair value
Trading Securities [Abstract]
Due less than one year	61,845	59,553
Due after 1 through 5 years	365,639	328,997
Due after 5 through 10 years	581,916	521,505
Due after 10 years	643,744	661,026
Subtotal	1,653,144	1,571,081
Mortgage and asset-backed securities	46,964	45,933
Fixed maturities, trading at fair value	1,700,108	1,617,014
Life Funds Withheld Assets | Fair value | Residential mortgage-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	930	961
Life Funds Withheld Assets | Fair value | Commercial mortgage-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	5,550	5,526
Life Funds Withheld Assets | Fair value | Other asset-backed securities
Trading Securities [Abstract]
Mortgage and asset-backed securities	$ 40,484	$ 39,446