Derivative Instruments and Hedging Activities Schedule of Interest Rate Swaptions (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional	$ 49,205,497,000	$ 36,528,169,000	$ 31,226,878,000
Interest Rate Swaption [Member] | Long [Member] | Variable Income Interest Rate [Member]
Derivative [Line Items]
Cost Basis	(24,700,000)	(13,255,000)
Fair Value	$ 16,095,000	$ 8,422,000
Weighted Average Remaining Maturity	3 months 6 days	7 months 28 days
Interest Rate Swaption [Member] | Long [Member] | Variable Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Cost Basis	$ (24,700,000)	$ (4,855,000)
Fair Value	$ 16,095,000	$ 2,430,000
Weighted Average Remaining Maturity	3 months 6 days	5 months 4 days
Interest Rate Swaption [Member] | Long [Member] | Variable Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member]
Derivative [Line Items]
Cost Basis		$ (8,400,000)
Fair Value		$ 5,992,000
Weighted Average Remaining Maturity		8 months 18 days
Interest Rate Swaption [Member] | Long [Member] | Fixed Income Interest Rate [Member]
Derivative [Line Items]
Cost Basis	$ (4,100,000)
Fair Value	$ 342,000
Weighted Average Remaining Maturity	1 month 3 days
Interest Rate Swaption [Member] | Long [Member] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Cost Basis	$ (4,100,000)
Fair Value	$ 342,000
Weighted Average Remaining Maturity	1 month 3 days
Interest Rate Swaption [Member] | Short [Member] | Fixed Income Interest Rate [Member]
Derivative [Line Items]
Cost Basis	$ (20,800,000)	$ (13,255,000)
Fair Value	$ (8,636,000)	$ 21,878,000
Weighted Average Remaining Maturity	3 months 7 days	7 months 16 days
Interest Rate Swaption [Member] | Short [Member] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Cost Basis	$ (20,800,000)	$ (4,855,000)
Fair Value	$ (8,636,000)	$ 9,001,000
Weighted Average Remaining Maturity	3 months 7 days	4 months 24 days
Interest Rate Swaption [Member] | Short [Member] | Fixed Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member]
Derivative [Line Items]
Cost Basis		$ (8,400,000)
Fair Value		$ 12,877,000
Weighted Average Remaining Maturity		8 months 18 days
Underlying Swap [Member] | Long [Member] | Variable Income Interest Rate [Member]
Derivative [Line Items]
Notional	$ 7,525,000,000	$ 1,700,000,000
Weighted Average Remaining Maturity	10 years	10 years
Underlying Swap [Member] | Long [Member] | Variable Income Interest Rate [Member] | London Interbank Offered Rate (LIBOR) [Member]
Derivative [Line Items]
Weighted Average Fixed Interest Rate	2.27%	3.15%
Underlying Swap [Member] | Long [Member] | Variable Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Notional	$ 7,525,000,000	$ 900,000,000
Weighted Average Remaining Maturity	10 years	10 years
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	2.27%	3.16%
Underlying Swap [Member] | Long [Member] | Variable Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member]
Derivative [Line Items]
Notional		$ 800,000,000
Weighted Average Remaining Maturity		10 years
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		3.14%
Underlying Swap [Member] | Long [Member] | Fixed Income Interest Rate [Member]
Derivative [Line Items]
Notional	$ 500,000,000
Weighted Average Remaining Maturity	10 years
Underlying Swap [Member] | Long [Member] | Fixed Income Interest Rate [Member] | London Interbank Offered Rate (LIBOR) [Member]
Derivative [Line Items]
Weighted Average Fixed Interest Rate	1.55%
Underlying Swap [Member] | Long [Member] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Notional	$ 500,000,000
Weighted Average Remaining Maturity	10 years
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	1.55%
Underlying Swap [Member] | Short [Member] | Fixed Income Interest Rate [Member]
Derivative [Line Items]
Notional	$ 6,768,000,000	$ 1,637,000,000
Weighted Average Remaining Maturity	10 years	10 years
Underlying Swap [Member] | Short [Member] | Fixed Income Interest Rate [Member] | London Interbank Offered Rate (LIBOR) [Member]
Derivative [Line Items]
Weighted Average Fixed Interest Rate	1.28%	2.65%
Underlying Swap [Member] | Short [Member] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Notional	$ 6,768,000,000	$ 845,000,000
Weighted Average Remaining Maturity	10 years	10 years
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	1.28%	2.66%
Underlying Swap [Member] | Short [Member] | Fixed Income Interest Rate [Member] | Six Months or Longer Remaining Maturity [Member]
Derivative [Line Items]
Notional		$ 792,000,000
Weighted Average Remaining Maturity		10 years
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		2.64%