Derivative Instruments and Hedging Activities Schedule of Total Return Swaps (Details) - USD ($)	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional	$ 41,330,283,000	$ 28,618,861,000	$ 31,226,878,000	$ 24,113,321,000	$ 16,988,116,000	$ 18,802,500,000
Total Return Swap [Member]
Derivative [Line Items]
Notional	49,691,000		63,507,000
Fair Value	2,000		(106,000)
Cost Basis	(59,000)		567,000
Derivative, Unrealized Gains (Losses)	61,000		(673,000)
Total Return Swap [Member] | Maturity Date, 1/12/2043 [Member]
Derivative [Line Items]
Notional	22,010,000		24,362,000
Fair Value	(1,000)		(24,000)
Cost Basis	(30,000)		201,000
Derivative, Unrealized Gains (Losses)	29,000		(225,000)
Total Return Swap [Member] | Maturity Date, 1/12/2044 [Member]
Derivative [Line Items]
Notional	27,681,000		39,145,000
Fair Value	3,000		(82,000)
Cost Basis	(29,000)		366,000
Derivative, Unrealized Gains (Losses)	$ 32,000		$ (448,000)