Derivative Instruments and Hedging Activities Schedule of Interest Rate Swaptions (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Jun. 30, 2018	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional	$ (41,330,283,000)	$ (31,226,878,000)	$ (28,618,861,000)	$ (24,113,321,000)	$ (16,988,116,000)	$ (18,802,500,000)
Interest Rate Swaption [Member] | Long [Member] | Variable Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Cost Basis	(9,400,000)	(21,380,000)
Fair Value	$ 30,608,000	$ 17,736,000
Weighted Average Remaining Maturity	3 months 19 days	4 months 1 day
Interest Rate Swaption [Member] | Long [Member] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Cost Basis		$ (4,660,000)
Fair Value		$ 2,982,000
Weighted Average Remaining Maturity		3 months 22 days
Interest Rate Swaption [Member] | Short [Member] | Variable Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Cost Basis		$ (7,950,000)
Fair Value		$ 5,619,000
Weighted Average Remaining Maturity		4 months 20 days
Interest Rate Swaption [Member] | Short [Member] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Cost Basis	$ (9,730,000)	$ (16,260,000)
Fair Value	$ (5,696,000)	$ 4,694,000
Weighted Average Remaining Maturity	3 months 27 days	5 months 5 days
Underlying Swap [Member] | Long [Member] | Variable Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Weighted Average Remaining Maturity	7 years 7 months 6 days	3 years 9 months 18 days
Notional	$ (5,225,000,000)	$ (7,200,000,000)
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	3.20%	2.27%
Underlying Swap [Member] | Long [Member] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Weighted Average Remaining Maturity		10 years
Notional		$ (2,300,000,000)
Weighted Average Fixed Interest Rate		2.10%
Underlying Swap [Member] | Short [Member] | Variable Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Weighted Average Remaining Maturity		10 years
Notional		$ (1,693,000,000)
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		2.70%
Underlying Swap [Member] | Short [Member] | Fixed Income Interest Rate [Member] | Less Than Six Months Remaining Maturity [Member]
Derivative [Line Items]
Weighted Average Remaining Maturity	7 years 8 months 12 days	5 years 7 months 6 days
Notional	$ (5,061,000,000)	$ (5,141,000,000)
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	2.70%	1.89%