Investments - Contractual maturities of available for sale securities (Details) - USD ($) $ in Thousands	Sep. 30, 2015		Dec. 31, 2014
Fixed Maturities - AFS
Amortized cost of fixed maturities	$ 32,207,019		$ 27,728,771
Fixed maturities, at fair value	33,276,322		29,359,034
Trading Securities, Debt, Amortized Cost	933,872		1,180
Trading Securities Debt Maturities And Short Term Investments	914,337		1,171
Excluding Life Funds Withheld Assets
Fixed Maturities - AFS
Due less than one year, amortized cost	2,235,684		1,972,224
Due less than one year, fair value	2,240,138		1,980,429
Due after 1 through 5 years, amortized cost	12,528,098		8,919,037
Due after 1 through 5 years, fair value	12,699,855		9,113,651
Due after 5 through 10 years, amortized cost	5,429,115		4,232,396
Due after 5 through 10 years, fair value	5,518,600		4,412,569
Due after 10 years, amortized cost	1,439,435		1,494,315
Due after 10 years, fair value	1,537,069		1,705,022
Subtotal - amortized cost	21,632,332		16,617,972
Subtotal - fair value	21,995,662		17,211,671
Mortgage and asset backed securities - amortized cost	7,512,068		6,809,460
Mortgage and asset backed securities - fair value	7,642,299		6,965,798
Amortized cost of fixed maturities	29,144,400		23,427,432
Fixed maturities, at fair value	29,637,961		24,177,469
Excluding Life Funds Withheld Assets | RMBS - Agency
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	3,831,823		3,625,171
Mortgage and asset backed securities - fair value	3,923,489		3,728,576
Amortized cost of fixed maturities	3,831,823		3,625,171
Fixed maturities, at fair value	3,923,489		3,728,576
Excluding Life Funds Withheld Assets | RMBS - Non-Agency
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	351,793		404,398
Mortgage and asset backed securities - fair value	362,965		427,351
Amortized cost of fixed maturities	351,793		404,398
Fixed maturities, at fair value	362,965		427,351
Excluding Life Funds Withheld Assets | Commercial mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	837,416		1,033,819
Mortgage and asset backed securities - fair value	850,084		1,052,544
Amortized cost of fixed maturities	837,416		1,033,819
Fixed maturities, at fair value	850,084		1,052,544
Excluding Life Funds Withheld Assets | Collateralized debt obligations
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	419,019		717,544
Mortgage and asset backed securities - fair value	410,163		692,034
Amortized cost of fixed maturities	419,019		717,544
Fixed maturities, at fair value	410,163		692,034
Excluding Life Funds Withheld Assets | Other asset-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	2,072,017		1,028,528
Mortgage and asset backed securities - fair value	2,095,598		1,065,293
Amortized cost of fixed maturities	2,072,017	[1]	1,028,528	[2]
Fixed maturities, at fair value	2,095,598	[1]	1,065,293	[2]
Life Funds Withheld Assets
Fixed Maturities - AFS
Due less than one year, amortized cost	93,671		117,048
Due less than one year, fair value	104,481		125,326
Due after 1 through 5 years, amortized cost	441,114		638,526
Due after 1 through 5 years, fair value	472,560		685,787
Due after 5 through 10 years, amortized cost	585,388		1,004,698
Due after 5 through 10 years, fair value	671,738		1,165,348
Due after 10 years, amortized cost	1,653,413		2,059,637
Due after 10 years, fair value	2,055,898		2,649,279
Subtotal - amortized cost	2,773,586		3,819,909
Subtotal - fair value	3,304,677		4,625,740
Mortgage and asset backed securities - amortized cost	289,033		481,430
Mortgage and asset backed securities - fair value	333,684		555,825
Amortized cost of fixed maturities	3,062,619		4,301,339
Fixed maturities, at fair value	3,638,361		5,181,565
Trading Securities, Debt, Amortized Cost	1,004,755		1,180
Trading Securities Debt Maturities And Short Term Investments	985,223		1,171
Life Funds Withheld Assets | RMBS - Agency
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	652		3,301
Mortgage and asset backed securities - fair value	814		3,782
Amortized cost of fixed maturities	652		3,301
Fixed maturities, at fair value	814		3,782
Life Funds Withheld Assets | RMBS - Non-Agency
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	25,341		71,075
Mortgage and asset backed securities - fair value	28,833		85,335
Amortized cost of fixed maturities	25,341		71,075
Fixed maturities, at fair value	28,833		85,335
Life Funds Withheld Assets | Commercial mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	121,266		168,886
Mortgage and asset backed securities - fair value	141,166		193,167
Amortized cost of fixed maturities	121,266		168,886
Fixed maturities, at fair value	141,166		193,167
Trading Securities, Debt, Amortized Cost	5,002
Trading Securities Debt Maturities And Short Term Investments	4,982
Life Funds Withheld Assets | Other asset-backed securities
Fixed Maturities - AFS
Mortgage and asset backed securities - amortized cost	141,774		238,168
Mortgage and asset backed securities - fair value	162,871		273,541
Amortized cost of fixed maturities	141,774		238,168
Fixed maturities, at fair value	162,871		273,541
Trading Securities, Debt, Amortized Cost	20,119
Trading Securities Debt Maturities And Short Term Investments	19,872
Life Funds Withheld Assets | Cost
Fixed Maturities - AFS
Due less than one year	71,135		0
Due after 1 through 5 years	239,984		0
Due after 5 through 10 years	178,020		1,180
Due after 10 years	490,495		0
Subtotal	979,634		1,180
Mortgage and asset-backed securities	25,121		0
Trading Securities, Debt, Amortized Cost	1,004,755		1,180
Life Funds Withheld Assets | Cost | Commercial mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset-backed securities	5,002		0
Life Funds Withheld Assets | Cost | Other asset-backed securities
Fixed Maturities - AFS
Mortgage and asset-backed securities	20,119		0
Life Funds Withheld Assets | Fair value
Fixed Maturities - AFS
Due less than one year	71,139		0
Due after 1 through 5 years	233,244		0
Due after 5 through 10 years	176,077		1,171
Due after 10 years	479,909		0
Subtotal	960,369		1,171
Mortgage and asset-backed securities	24,854		0
Trading Securities Debt Maturities And Short Term Investments	985,223		1,171
Life Funds Withheld Assets | Fair value | Commercial mortgage-backed securities
Fixed Maturities - AFS
Mortgage and asset-backed securities	4,982		0
Life Funds Withheld Assets | Fair value | Other asset-backed securities
Fixed Maturities - AFS
Mortgage and asset-backed securities	$ 19,872		$ 0

[1] Included are certain medium term notes supported primarily by pools of European investment grade credit with varying degrees of leverage. The notes have a fair value of $74.3 million and an amortized cost of $64.6 million. These notes allow the investor to participate in cash flows of the underlying bonds including certain residual values, which could serve to either decrease or increase the ultimate value of these notes.

[2] Included are certain medium term notes supported primarily by pools of European investment grade credit with varying degrees of leverage. The notes have a fair value of $79.9 million and an amortized cost of $68.4 million. These notes allow the investor to participate in cash flows of the underlying bonds including certain residual values, which could serve to either decrease or increase the ultimate value of these notes.