Schedule of Derivative Instruments in Statement of Financial Position, Fair Value (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ (60,549,307,000)	$ (48,445,497,000)	$ (58,777,783,000)	$ (36,528,169,000)
Inverse Interest-Only Securities [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	73,904,000	69,469,000
U.S. Treasury Futures [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	(875,000,000.0)	(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	117,368,000	188,051,000
Notional Disclosures [Abstract]
Derivative, Notional Amount	(3,944,239,000)	(14,343,137,000)
Derivative Financial Instruments, Assets [Member] | Inverse Interest-Only Securities [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	(379,239,000)	(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	0	7,801,000
Notional Disclosures [Abstract]
Derivative, Notional Amount	0	(1,257,000,000)
Derivative Financial Instruments, Assets [Member] | TBAs [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	43,464,000	8,011,000
Notional Disclosures [Abstract]
Derivative, Notional Amount	(3,565,000,000)	(9,584,000,000)
Derivative Financial Instruments, Assets [Member] | U.S. Treasury Futures [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	0	502,000
Notional Disclosures [Abstract]
Derivative, Notional Amount	0	(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	(176,156,000)	(6,740,000)
Notional Disclosures [Abstract]
Derivative, Notional Amount	(56,605,068,000)	(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	(56,158,068,000)	(36,977,470,000)
Derivative Financial Instruments, Liabilities [Member] | Interest Rate Swaption [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	(62,713,000)	0
Notional Disclosures [Abstract]
Derivative, Notional Amount	(1,376,000,000)	0
Derivative Financial Instruments, Liabilities [Member] | TBAs [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	(109,426,000)	(6,711,000)
Notional Disclosures [Abstract]
Derivative, Notional Amount	(1,804,000,000)	(2,157,000,000)
Derivative Financial Instruments, Liabilities [Member] | U.S. Treasury Futures [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	(4,017,000)	0
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ (875,000,000)	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)