Derivative Instruments and Hedging Activities Schedule of Interest Rate Swap Payers Associated with the Investment Portfolio (Details) $ in Thousands	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Derivative [Line Items]
Derivative, Notional Amount	$ 22,062,904	$ 26,141,370	$ 21,998,381	$ 28,317,580	$ 27,401,335	$ 48,115,523
Interest Rate Swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	16,945,430	$ 13,697,000	14,268,806	$ 14,025,523	$ 16,225,523	$ 18,584,000
Interest Rate Risk Associated with the Investment Portfolio [Member] | Interest Rate Swap [Member] | Long [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 6,010,561		$ 3,250,000
Derivative, Average Fixed Interest Rate	1.456%		1.787%
Derivative, Average Variable Interest Rate	0.843%		0.503%
Derivative, Average Remaining Period Until Maturity	2.53		3.74
Interest Rate Risk Associated with the Investment Portfolio [Member] | Derivative Maturity Over Two And Within Three Years From Balance Sheet Date [Member] | Interest Rate Swap [Member] | Long [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 4,669,049		$ 2,040,000
Derivative, Average Fixed Interest Rate	1.273%		1.563%
Derivative, Average Variable Interest Rate	0.853%		0.487%
Derivative, Average Remaining Period Until Maturity	1.85		2.94
Interest Rate Risk Associated with the Investment Portfolio [Member] | Derivative Maturity Over Four Years From Balance Sheet Date [Member] | Interest Rate Swap [Member] | Long [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 1,341,512		$ 1,210,000
Derivative, Average Fixed Interest Rate	2.094%		2.164%
Derivative, Average Variable Interest Rate	0.812%		0.531%
Derivative, Average Remaining Period Until Maturity	4.88		5.08