Investments Maturity profile (Details) - Trading securities [Member] $ in Thousands	Dec. 31, 2015 USD ($)
Amortized Cost
Maturity Profile [Line Items]
One year or less	$ 556,422
More than one year through five years	4,552,338
More than five years through ten years	3,336,663
More than ten years	1,538,101
Subtotal	9,983,524
Mortgage/asset-backed securities	3,376,984
Total	13,360,508
Fair Value
Maturity Profile [Line Items]
One year or less	555,956
More than one year through five years	4,608,744
More than five years through ten years	3,341,696
More than ten years	1,610,587
Subtotal	10,116,983
Mortgage/asset-backed securities	3,377,967
Total	$ 13,494,950