Schedule of Derivative Instruments in Statement of Financial Position, Fair Value (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 49,205,497,000	$ 36,528,169,000	$ 31,226,878,000
Inverse Interest-Only Securities [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	69,469,000	70,813,000
Interest Rate Cap [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount		2,500,000,000	$ 0
Short US Treasuries [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value		781,500,000
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	383,000
Notional Disclosures [Abstract]
Derivative, Notional Amount	41,890,000	48,265,000
Derivative Financial Instruments, Assets [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	188,051,000	319,981,000
Notional Disclosures [Abstract]
Derivative, Notional Amount	14,343,137,000	36,258,904,000
Derivative Financial Instruments, Assets [Member] | Inverse Interest-Only Securities [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	397,137,000	476,299,000
Derivative Financial Instruments, Assets [Member] | Interest Rate Swap [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	102,268,000	187,231,000
Notional Disclosures [Abstract]
Derivative, Notional Amount	2,725,000,000	26,798,605,000
Derivative Financial Instruments, Assets [Member] | Interest Rate Cap [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value		40,335,000
Notional Disclosures [Abstract]
Derivative, Notional Amount		2,500,000,000
Derivative Financial Instruments, Assets [Member] | Interest Rate Swaption [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	7,801,000	0
Notional Disclosures [Abstract]
Derivative, Notional Amount	1,257,000,000	0
Derivative Financial Instruments, Assets [Member] | TBAs [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	8,011,000	21,602,000
Notional Disclosures [Abstract]
Derivative, Notional Amount	9,584,000,000	6,484,000,000
Derivative Financial Instruments, Assets [Member] | Put and Call Options for TBAs [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value		0
Notional Disclosures [Abstract]
Derivative, Notional Amount		0
Derivative Financial Instruments, Assets [Member] | Short US Treasuries [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value		0
Notional Disclosures [Abstract]
Derivative, Notional Amount		0
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	0
Notional Disclosures [Abstract]
Derivative, Notional Amount	0	0
Derivative Financial Instruments, Liabilities [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	(6,740,000)	(820,590,000)
Notional Disclosures [Abstract]
Derivative, Notional Amount	34,862,360,000	5,403,265,000
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	36,977,470,000	2,725,000,000
Derivative Financial Instruments, Liabilities [Member] | Interest Rate Cap [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value		0
Notional Disclosures [Abstract]
Derivative, Notional Amount		0
Derivative Financial Instruments, Liabilities [Member] | Interest Rate Swaption [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	0	(13,456,000)
Notional Disclosures [Abstract]
Derivative, Notional Amount	0	63,000,000
Derivative Financial Instruments, Liabilities [Member] | TBAs [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	(6,711,000)	0
Notional Disclosures [Abstract]
Derivative, Notional Amount	(2,157,000,000)	0
Derivative Financial Instruments, Liabilities [Member] | Put and Call Options for TBAs [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value		(25,296,000)
Notional Disclosures [Abstract]
Derivative, Notional Amount		1,767,000,000
Derivative Financial Instruments, Liabilities [Member] | Short US Treasuries [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value		(781,455,000)
Notional Disclosures [Abstract]
Derivative, Notional Amount		800,000,000.0
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)	(383,000)
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 41,890,000	$ 48,265,000