Derivative Instruments and Hedging Activities Schedule of Interest Rate Swaptions (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	Jun. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Derivative [Line Items]
Notional	$ 24,113,321,000	$ 18,802,500,000	$ 16,988,116,000	$ 22,062,904,000	$ 26,141,370,000	$ 21,998,381,000
Underlying Swap [Member] | Long [Member] | Variable Income Interest Rate [Member]
Derivative [Line Items]
Weighted Average Remaining Maturity	5 years	5 years 1 month 8 days
Notional	$ 3,225,000,000	$ 4,800,000,000
Underlying Swap [Member] | Long [Member] | Variable Income Interest Rate [Member] | London Interbank Offered Rate (LIBOR) [Member]
Derivative [Line Items]
Weighted Average Fixed Interest Rate	2.25%	2.24%
Underlying Swap [Member] | Long [Member] | Variable Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Weighted Average Remaining Maturity	5 years	4 years 9 months 10 days
Notional	$ 3,225,000,000	$ 4,500,000,000
Underlying Swap [Member] | Long [Member] | Variable Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member] | London Interbank Offered Rate (LIBOR) [Member]
Derivative [Line Items]
Weighted Average Fixed Interest Rate	2.25%	2.16%
Underlying Swap [Member] | Long [Member] | Variable Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member]
Derivative [Line Items]
Weighted Average Remaining Maturity		10 years
Notional		$ 300,000,000
Underlying Swap [Member] | Long [Member] | Variable Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member] | London Interbank Offered Rate (LIBOR) [Member]
Derivative [Line Items]
Weighted Average Fixed Interest Rate		3.50%
Underlying Swap [Member] | Long [Member] | Fixed Income Interest Rate [Member]
Derivative [Line Items]
Weighted Average Remaining Maturity	8 years 1 month 10 days
Notional	$ 4,820,000,000
Underlying Swap [Member] | Long [Member] | Fixed Income Interest Rate [Member] | London Interbank Offered Rate (LIBOR) [Member]
Derivative [Line Items]
Weighted Average Fixed Interest Rate	1.98%
Underlying Swap [Member] | Long [Member] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Weighted Average Remaining Maturity	8 years 3 days
Notional	$ 4,570,000,000
Underlying Swap [Member] | Long [Member] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member] | London Interbank Offered Rate (LIBOR) [Member]
Derivative [Line Items]
Weighted Average Fixed Interest Rate	1.96%
Underlying Swap [Member] | Long [Member] | Fixed Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member]
Derivative [Line Items]
Weighted Average Remaining Maturity	10 years
Notional	$ 250,000,000
Underlying Swap [Member] | Long [Member] | Fixed Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member] | London Interbank Offered Rate (LIBOR) [Member]
Derivative [Line Items]
Weighted Average Fixed Interest Rate	2.35%
Underlying Swap [Member] | Short [Member] | Variable Income Interest Rate [Member]
Derivative [Line Items]
Weighted Average Remaining Maturity	5 years	10 years
Notional	$ 600,000,000	$ 800,000,000
Underlying Swap [Member] | Short [Member] | Variable Income Interest Rate [Member] | London Interbank Offered Rate (LIBOR) [Member]
Derivative [Line Items]
Weighted Average Fixed Interest Rate	2.42%	3.44%
Underlying Swap [Member] | Short [Member] | Variable Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Weighted Average Remaining Maturity	5 years	10 years
Notional	$ 600,000,000	$ 500,000,000
Underlying Swap [Member] | Short [Member] | Variable Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member] | London Interbank Offered Rate (LIBOR) [Member]
Derivative [Line Items]
Weighted Average Fixed Interest Rate	2.42%	3.40%
Underlying Swap [Member] | Short [Member] | Variable Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member]
Derivative [Line Items]
Weighted Average Remaining Maturity		10 years
Notional		$ 300,000,000
Underlying Swap [Member] | Short [Member] | Variable Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member] | London Interbank Offered Rate (LIBOR) [Member]
Derivative [Line Items]
Weighted Average Fixed Interest Rate		3.50%
Underlying Swap [Member] | Short [Member] | Fixed Income Interest Rate [Member]
Derivative [Line Items]
Weighted Average Remaining Maturity	5 years 8 months 3 days	4 years 11 months 8 days
Notional	$ 4,631,000,000	$ 3,775,000,000
Underlying Swap [Member] | Short [Member] | Fixed Income Interest Rate [Member] | London Interbank Offered Rate (LIBOR) [Member]
Derivative [Line Items]
Weighted Average Fixed Interest Rate	1.75%	1.19%
Underlying Swap [Member] | Short [Member] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Weighted Average Remaining Maturity	5 years	4 years 11 months 8 days
Notional	$ 4,006,000,000	$ 3,775,000,000
Underlying Swap [Member] | Short [Member] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member] | London Interbank Offered Rate (LIBOR) [Member]
Derivative [Line Items]
Weighted Average Fixed Interest Rate	1.72%	1.19%
Underlying Swap [Member] | Short [Member] | Fixed Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member]
Derivative [Line Items]
Weighted Average Remaining Maturity	10 years
Notional	$ 625,000,000
Underlying Swap [Member] | Short [Member] | Fixed Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member] | London Interbank Offered Rate (LIBOR) [Member]
Derivative [Line Items]
Weighted Average Fixed Interest Rate	1.95%
Interest Rate Swaption [Member] | Long [Member] | Variable Income Interest Rate [Member]
Derivative [Line Items]
Cost Basis	$ 9,260,000	$ 43,015,000
Fair Value	$ 6,295,000	$ 42,941,000
Weighted Average Remaining Maturity	3 months 26 days	1 month 7 days
Interest Rate Swaption [Member] | Long [Member] | Variable Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Cost Basis	$ 9,260,000	$ 29,360,000
Fair Value	$ 6,295,000	$ 42,149,000
Weighted Average Remaining Maturity	3 months 26 days	1 month 7 days
Interest Rate Swaption [Member] | Long [Member] | Variable Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member]
Derivative [Line Items]
Cost Basis		$ 13,655,000
Fair Value		$ 792,000
Weighted Average Remaining Maturity		6 months 21 days
Interest Rate Swaption [Member] | Long [Member] | Fixed Income Interest Rate [Member]
Derivative [Line Items]
Cost Basis	$ 17,570,000
Fair Value	$ 12,206,000
Weighted Average Remaining Maturity	3 months 2 days
Interest Rate Swaption [Member] | Long [Member] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Cost Basis	$ 17,570,000
Fair Value	$ 7,716,000
Weighted Average Remaining Maturity	2 months 10 days
Interest Rate Swaption [Member] | Long [Member] | Fixed Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member]
Derivative [Line Items]
Cost Basis	$ 0
Fair Value	$ 4,490,000
Weighted Average Remaining Maturity	7 months 24 days
Interest Rate Swaption [Member] | Short [Member] | Variable Income Interest Rate [Member]
Derivative [Line Items]
Cost Basis	$ 0	$ 81,248,000
Fair Value	$ 0	$ 2,352,000
Weighted Average Remaining Maturity	11 days	6 months 1 day
Interest Rate Swaption [Member] | Short [Member] | Variable Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Cost Basis	$ 0	$ 51,355,000
Fair Value	$ 0	$ 1,414,000
Weighted Average Remaining Maturity	11 days	5 months 25 days
Interest Rate Swaption [Member] | Short [Member] | Variable Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member]
Derivative [Line Items]
Cost Basis		$ 29,893,000
Fair Value		$ 938,000
Weighted Average Remaining Maturity		6 months 23 days
Interest Rate Swaption [Member] | Short [Member] | Fixed Income Interest Rate [Member]
Derivative [Line Items]
Cost Basis	$ 10,660,000	$ 0
Fair Value	$ (9,106,000)	$ 2,353,000
Weighted Average Remaining Maturity	4 months 9 days	2 months 9 days
Interest Rate Swaption [Member] | Short [Member] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Cost Basis	$ 9,260,000	$ 0
Fair Value	$ (5,257,000)	$ 2,353,000
Weighted Average Remaining Maturity	3 years 9 months 9 days	2 months 9 days
Interest Rate Swaption [Member] | Short [Member] | Fixed Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member]
Derivative [Line Items]
Cost Basis	$ 1,400,000
Fair Value	$ (3,849,000)
Weighted Average Remaining Maturity	7 months 24 days