Schedule of Derivative Instruments in Statement of Financial Position, Fair Value (Details) - USD ($)	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 8,076,933,000	$ 60,549,307,000	$ 48,445,497,000	$ 55,549,424,000	$ 58,777,783,000	$ 36,528,169,000
Inverse Interest-Only Securities [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	72,822,000		69,469,000
U.S. Treasury Futures [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount			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	110,527,000		188,051,000
Notional Disclosures [Abstract]
Derivative, Notional Amount	2,397,933,000		14,343,137,000
Derivative Financial Instruments, Assets [Member] | Inverse Interest-Only Securities [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	361,933,000		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			7,801,000
Notional Disclosures [Abstract]
Derivative, Notional Amount			1,257,000,000
Derivative Financial Instruments, Assets [Member] | TBAs [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	37,705,000		8,011,000
Notional Disclosures [Abstract]
Derivative, Notional Amount	2,036,000,000		9,584,000,000
Derivative Financial Instruments, Assets [Member] | U.S. Treasury Futures [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value			502,000
Notional Disclosures [Abstract]
Derivative, Notional Amount			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	(1,298,000)		(6,740,000)
Notional Disclosures [Abstract]
Derivative, Notional Amount	5,679,000,000		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	4,479,000,000		36,977,470,000
Derivative Financial Instruments, Liabilities [Member] | Interest Rate Swaption [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value			0
Notional Disclosures [Abstract]
Derivative, Notional Amount			0
Derivative Financial Instruments, Liabilities [Member] | TBAs [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	(1,298,000)		(6,711,000)
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 1,200,000,000		2,157,000,000
Derivative Financial Instruments, Liabilities [Member] | U.S. Treasury Futures [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value			0
Notional Disclosures [Abstract]
Derivative, Notional Amount			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