Derivative Instruments and Hedging Activities Schedule of Interest Rate Swap Receivers Associated with the Investment Portfolio (Details) $ in Thousands	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Derivative [Line Items]
Derivative, Notional Amount	$ 18,802,500	$ 21,998,381	$ 48,115,523
Interest Rate Swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	20,371,063	14,268,806	$ 18,584,000
Interest Rate Swap [Member] | Interest Rate Risk Associated with the Investment Portfolio [Member] | Short [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 5,064,000	$ 3,164,000
Derivative, Average Variable Interest Rate	1.894%	2.145%
Derivative, Average Fixed Interest Rate	0.941%	0.431%
Derivative, Average Remaining Period Until Maturity	4.57	6.26
Interest Rate Swap [Member] | Interest Rate Risk Associated with the Investment Portfolio [Member] | Derivative Maturity Over One And Within Two Years From Balance Sheet Date [Member] | Short [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 575,000
Derivative, Average Variable Interest Rate	1.44%
Derivative, Average Fixed Interest Rate	0.911%
Derivative, Average Remaining Period Until Maturity	1.89
Interest Rate Swap [Member] | Interest Rate Risk Associated with the Investment Portfolio [Member] | Derivative Maturity Over Two And Within Three Years From Balance Sheet Date [Member] | Short [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 500,000	$ 575,000
Derivative, Average Variable Interest Rate	1.042%	1.44%
Derivative, Average Fixed Interest Rate	0.882%	0.329%
Derivative, Average Remaining Period Until Maturity	2.06	2.89
Interest Rate Swap [Member] | Interest Rate Risk Associated with the Investment Portfolio [Member] | Derivative Maturity Over Three And Within Four Years From Balance Sheet Date [Member] | Short [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 510,000
Derivative, Average Variable Interest Rate	1.58%
Derivative, Average Fixed Interest Rate	0.881%
Derivative, Average Remaining Period Until Maturity	3.59
Interest Rate Swap [Member] | Interest Rate Risk Associated with the Investment Portfolio [Member] | Derivative Maturity Over Four Years From Balance Sheet Date [Member] | Short [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 3,479,000	$ 2,589,000
Derivative, Average Variable Interest Rate	2.137%	2.301%
Derivative, Average Fixed Interest Rate	0.963%	0.453%
Derivative, Average Remaining Period Until Maturity	5.52	7.00