Derivative Instruments and Hedging Activities Schedule of Interest Rate Swap Payers Associated with the Investment Portfolio (Details) $ in Thousands	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Derivative [Line Items]
Derivative, Notional Amount	$ 26,141,370	$ 26,390,596	$ 21,998,381	$ 27,401,335	$ 33,709,043	$ 48,115,523
Interest Rate Swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	13,697,000	$ 15,425,513	14,268,806	$ 16,225,523	$ 19,929,000	$ 18,584,000
Interest Rate Risk Associated with the Investment Portfolio [Member] | Interest Rate Swap [Member] | Long [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 5,250,000		$ 3,250,000
Derivative, Average Fixed Interest Rate	1.504%		1.787%
Derivative, Average Variable Interest Rate	0.651%		0.503%
Derivative, Average Remaining Period Until Maturity	2.66		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	$ 4,040,000		$ 2,040,000
Derivative, Average Fixed Interest Rate	1.307%		1.563%
Derivative, Average Variable Interest Rate	0.654%		0.487%
Derivative, Average Remaining Period Until Maturity	2.08		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	$ 1,210,000		$ 1,210,000
Derivative, Average Fixed Interest Rate	2.164%		2.164%
Derivative, Average Variable Interest Rate	0.638%		0.531%
Derivative, Average Remaining Period Until Maturity	4.58		5.08