Investments - Contractual maturities of income securities (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
AFS securities
Amortized cost of Fixed maturities		$ 30,157,581	$ 29,587,656
Fixed maturities, at fair value		30,928,988	30,256,602
Debt Securities Other Than Temporary Impairments Included In Other Comprehensive Income	[1]	43,646	51,135
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading		1,946,501	1,548,178
Fixed maturities, trading, at fair value		2,006,385	1,617,014
Exclude Life Funds Withheld Assets
AFS securities
Due less than one year, amortized cost		1,556,688	2,121,088
Due less than one year, fair value		1,564,360	2,135,262
Due after 1 through 5 years, amortized cost		12,243,590	12,400,798
Due after 1 through 5 years, fair value		12,309,732	12,463,844
Due after 5 through 10 years, amortized cost		6,268,217	5,509,194
Due after 5 through 10 years, fair value		6,362,314	5,531,402
Due after 10 years, amortized cost		1,048,129	1,335,687
Due after 10 years, fair value		1,187,688	1,406,853
Subtotal - amortized cost		21,116,624	21,366,767
Subtotal - fair value		21,424,094	21,537,361
Mortgage and asset-backed securities - amortized cost		7,452,906	6,402,162
Mortgage and asset-backed securities - fair value		7,477,782	6,411,199
Amortized cost of Fixed maturities		28,569,530	27,768,929
Fixed maturities, at fair value		28,901,876	27,948,560
Debt Securities Other Than Temporary Impairments Included In Other Comprehensive Income	[1]	43,646	51,135
Exclude Life Funds Withheld Assets | RMBS
AFS securities
Mortgage and asset-backed securities - amortized cost		4,717,542	4,479,722
Mortgage and asset-backed securities - fair value		4,724,402	4,492,625
Amortized cost of Fixed maturities		4,717,542	4,479,722
Fixed maturities, at fair value		4,724,402	4,492,625
Debt Securities Other Than Temporary Impairments Included In Other Comprehensive Income	[1]	8,795	47,879
Exclude Life Funds Withheld Assets | CMBS
AFS securities
Mortgage and asset-backed securities - amortized cost		1,246,406	670,005
Mortgage and asset-backed securities - fair value		1,244,017	665,186
Amortized cost of Fixed maturities		1,246,406	670,005
Fixed maturities, at fair value		1,244,017	665,186
Debt Securities Other Than Temporary Impairments Included In Other Comprehensive Income	[1]	817	1,191
Exclude Life Funds Withheld Assets | Other asset-backed securities
AFS securities
Mortgage and asset-backed securities - amortized cost		1,488,958	1,252,435
Mortgage and asset-backed securities - fair value		1,509,363	1,253,388
Amortized cost of Fixed maturities		1,488,958	1,252,435
Fixed maturities, at fair value		1,509,363	1,253,388
Debt Securities Other Than Temporary Impairments Included In Other Comprehensive Income	[1]	34,016	2,029
Life Funds Withheld Assets
AFS securities
Due less than one year, amortized cost		47,143	71,207
Due less than one year, fair value		49,233	80,749
Due after 1 through 5 years, amortized cost		286,524	289,710
Due after 1 through 5 years, fair value		313,227	318,605
Due after 5 through 10 years, amortized cost		168,897	274,727
Due after 5 through 10 years, fair value		205,536	324,759
Due after 10 years, amortized cost		849,739	1,001,189
Due after 10 years, fair value		1,165,067	1,365,749
Subtotal - amortized cost		1,352,303	1,636,833
Subtotal - fair value		1,733,063	2,089,862
Mortgage and asset-backed securities - amortized cost		235,748	181,894
Mortgage and asset-backed securities - fair value		294,049	218,180
Amortized cost of Fixed maturities		1,588,051	1,818,727
Fixed maturities, at fair value		2,027,112	2,308,042
Debt Securities Other Than Temporary Impairments Included In Other Comprehensive Income	[1]	0	0
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading		1,946,501	1,548,178
Fixed maturities, trading, at fair value		2,006,385	1,617,014
Life Funds Withheld Assets | RMBS
AFS securities
Mortgage and asset-backed securities - amortized cost		448	19,591
Mortgage and asset-backed securities - fair value		515	22,380
Amortized cost of Fixed maturities		448	19,591
Fixed maturities, at fair value		515	22,380
Debt Securities Other Than Temporary Impairments Included In Other Comprehensive Income		0	0
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading		1,020	936
Fixed maturities, trading, at fair value		932	961
Life Funds Withheld Assets | CMBS
AFS securities
Mortgage and asset-backed securities - amortized cost		97,356	77,762
Mortgage and asset-backed securities - fair value		122,272	94,240
Amortized cost of Fixed maturities		97,356	77,762
Fixed maturities, at fair value		122,272	94,240
Debt Securities Other Than Temporary Impairments Included In Other Comprehensive Income		0	0
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading		7,345	5,242
Fixed maturities, trading, at fair value		7,580	5,526
Life Funds Withheld Assets | Other asset-backed securities
AFS securities
Mortgage and asset-backed securities - amortized cost		137,944	84,541
Mortgage and asset-backed securities - fair value		171,262	101,560
Amortized cost of Fixed maturities		137,944	84,541
Fixed maturities, at fair value		171,262	101,560
Debt Securities Other Than Temporary Impairments Included In Other Comprehensive Income	[1]	0	0
Trading Securities [Abstract]
Amortized cost of fixed maturities, trading		39,771	38,342
Fixed maturities, trading, at fair value		41,337	39,446
Cost | Life Funds Withheld Assets
Trading Securities [Abstract]
Trading securities due less than one year		59,962	59,138
Trading securities due after 1 through 5 years		486,847	322,574
Trading securities due after 5 through 10 years		645,573	507,631
Trading securities due after 10 years		705,983	614,315
Trading Securities Debt Maturities And Short Term Investments Fixed Maturities		1,898,365	1,503,658
Trading Securities Debt Maturities Without Single Maturity Date		48,136	44,520
Amortized cost of fixed maturities, trading		1,946,501	1,548,178
Cost | Life Funds Withheld Assets | RMBS
Trading Securities [Abstract]
Trading Securities Debt Maturities Without Single Maturity Date		1,020	936
Cost | Life Funds Withheld Assets | CMBS
Trading Securities [Abstract]
Trading Securities Debt Maturities Without Single Maturity Date		7,345	5,242
Cost | Life Funds Withheld Assets | Other asset-backed securities
Trading Securities [Abstract]
Trading Securities Debt Maturities Without Single Maturity Date		39,771	38,342
Fair Value | Life Funds Withheld Assets
Trading Securities [Abstract]
Trading securities due less than one year		59,605	59,553
Trading securities due after 1 through 5 years		492,998	328,997
Trading securities due after 5 through 10 years		657,093	521,505
Trading securities due after 10 years		746,840	661,026
Trading Securities Debt Maturities And Short Term Investments Fixed Maturities		1,956,536	1,571,081
Trading Securities Debt Maturities Without Single Maturity Date		49,849	45,933
Fixed maturities, trading, at fair value		2,006,385	1,617,014
Fair Value | Life Funds Withheld Assets | RMBS
Trading Securities [Abstract]
Trading Securities Debt Maturities Without Single Maturity Date		932	961
Fair Value | Life Funds Withheld Assets | CMBS
Trading Securities [Abstract]
Trading Securities Debt Maturities Without Single Maturity Date		7,580	5,526
Fair Value | Life Funds Withheld Assets | Other asset-backed securities
Trading Securities [Abstract]
Trading Securities Debt Maturities Without Single Maturity Date		$ 41,337	$ 39,446

[1] Represents the non-credit component of OTTI losses, adjusted for subsequent sales of securities. It does not include the change in fair value subsequent to the impairment measurement date.