Schedule of Derivative Instruments in Statement of Financial Position, Fair Value (Details) - USD ($)	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 25,836,064,000	$ 8,076,933,000	$ 48,445,497,000	$ 54,226,205,000	$ 55,549,424,000	$ 36,528,169,000
Inverse Interest-Only Securities [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	67,468,000		69,469,000
U.S. Treasury Futures [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	866,600,000		380,000,000.0
Markit IOS Total Return Swap [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value			29,000
Notional Disclosures [Abstract]
Derivative, Notional Amount			41,890,000
Derivative Financial Instruments, Assets [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	97,889,000		188,051,000
Notional Disclosures [Abstract]
Derivative, Notional Amount	11,152,246,000		14,343,137,000
Derivative Financial Instruments, Assets [Member] | Inverse Interest-Only Securities [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	338,646,000		397,137,000
Derivative Financial Instruments, Assets [Member] | Interest Rate Swap [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	0		102,268,000
Notional Disclosures [Abstract]
Derivative, Notional Amount	0		2,725,000,000
Derivative Financial Instruments, Assets [Member] | Interest Rate Swaption [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	10,791,000		7,801,000
Notional Disclosures [Abstract]
Derivative, Notional Amount	6,000,000,000		1,257,000,000
Derivative Financial Instruments, Assets [Member] | TBAs [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	18,460,000		8,011,000
Notional Disclosures [Abstract]
Derivative, Notional Amount	3,947,000,000		9,584,000,000
Derivative Financial Instruments, Assets [Member] | U.S. Treasury Futures [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	1,170,000		502,000
Notional Disclosures [Abstract]
Derivative, Notional Amount	866,600,000		380,000,000
Derivative Financial Instruments, Assets [Member] | Markit IOS Total Return Swap [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value			0
Notional Disclosures [Abstract]
Derivative, Notional Amount			0
Derivative Financial Instruments, Liabilities [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	(3,551,000)		(6,740,000)
Notional Disclosures [Abstract]
Derivative, Notional Amount	14,683,818,000		34,862,360,000
Derivative Financial Instruments, Liabilities [Member] | Inverse Interest-Only Securities [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	0		0
Notional Disclosures [Abstract]
Derivative, Notional Amount	0		0
Derivative Financial Instruments, Liabilities [Member] | Interest Rate Swap [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	0		0
Notional Disclosures [Abstract]
Derivative, Notional Amount	12,394,818,000		36,977,470,000
Derivative Financial Instruments, Liabilities [Member] | Interest Rate Swaption [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	0		0
Notional Disclosures [Abstract]
Derivative, Notional Amount	0		0
Derivative Financial Instruments, Liabilities [Member] | TBAs [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	(3,551,000)		(6,711,000)
Notional Disclosures [Abstract]
Derivative, Notional Amount	2,289,000,000		2,157,000,000
Derivative Financial Instruments, Liabilities [Member] | U.S. Treasury Futures [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	0		0
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 0		0
Derivative Financial Instruments, Liabilities [Member] | Markit IOS Total Return Swap [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value			(29,000)
Notional Disclosures [Abstract]
Derivative, Notional Amount			$ 41,890,000