Derivative Instruments and Hedging Activities Schedule of Total Return Swaps (Details) - USD ($)	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
Markit IOS Total Return Swap [Member]
Derivative [Line Items]
Notional	41,890,000	48,265,000
Fair Value	(29,000)	(383,000)
Cost Basis	(59,000)	(59,000)
Derivative, Unrealized Gains (Losses)	30,000	(324,000)
Markit IOS Total Return Swap [Member] | Maturity Date, 1/12/2043 [Member]
Derivative [Line Items]
Notional	18,625,000	21,395,000
Fair Value	5,000	(153,000)
Cost Basis	(30,000)	(30,000)
Derivative, Unrealized Gains (Losses)	35,000	(123,000)
Markit IOS Total Return Swap [Member] | Maturity Date, 1/12/2044 [Member]
Derivative [Line Items]
Notional	23,265,000	26,870,000
Fair Value	(34,000)	(230,000)
Cost Basis	(29,000)	(29,000)
Derivative, Unrealized Gains (Losses)	$ (5,000)	$ (201,000)