Schedule of Derivative Instruments in Statement of Financial Position, Fair Value (Details) - USD ($)	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Notional Disclosures [Abstract]
Notional	$ 24,113,321,000	$ 16,988,116,000	$ 18,802,500,000	$ 22,062,904,000	$ 26,141,370,000	$ 21,998,381,000
Inverse Interest-Only Securities [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	102,235,000		127,843,000
Notional Disclosures [Abstract]
Notional	621,548,978		740,844,023
Total Return Swap [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	393,000		(17,000)
Notional Disclosures [Abstract]
Notional	65,895,000		90,593,000
Derivative Financial Instruments, Assets [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	238,305,000		324,182,000
Notional Disclosures [Abstract]
Notional	20,122,892,000		21,708,907,000
Derivative Financial Instruments, Assets [Member] | Inverse Interest-Only Securities [Member]
Notional Disclosures [Abstract]
Notional	621,548,978		740,844,023
Derivative Financial Instruments, Assets [Member] | Interest Rate Swap [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	120,423,000		109,531,000
Notional Disclosures [Abstract]
Notional	13,216,448,000		18,471,063,000
Derivative Financial Instruments, Assets [Member] | Interest Rate Swaption [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	9,395,000		39,881,000
Notional Disclosures [Abstract]
Notional	2,814,000,000		825,000,000
Derivative Financial Instruments, Assets [Member] | Forward Contracts [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	5,703,000		4,294,000
Notional Disclosures [Abstract]
Notional	1,405,000,000		536,000,000
Derivative Financial Instruments, Assets [Member] | Options Held [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	156,000		42,633,000
Notional Disclosures [Abstract]
Notional	2,000,000,000		1,136,000,000
Derivative Financial Instruments, Assets [Member] | Total Return Swap [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	393,000		0
Notional Disclosures [Abstract]
Notional	65,895,208		0
Derivative Financial Instruments, Liabilities [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	(11,312,000)		(12,501,000)
Notional Disclosures [Abstract]
Notional	6,800,429,000		3,543,593,000
Derivative Financial Instruments, Liabilities [Member] | Inverse Interest-Only Securities [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	0		0
Notional Disclosures [Abstract]
Notional	0		0
Derivative Financial Instruments, Liabilities [Member] | Interest Rate Swap [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	(11,312,000)		(495,000)
Notional Disclosures [Abstract]
Notional	6,800,429,000		1,900,000,000
Derivative Financial Instruments, Liabilities [Member] | Interest Rate Swaption [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	0		(1,645,000)
Notional Disclosures [Abstract]
Notional	0		600,000,000
Derivative Financial Instruments, Liabilities [Member] | Forward Contracts [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	0		(10,344,000)
Notional Disclosures [Abstract]
Notional	0		953,000,000
Derivative Financial Instruments, Liabilities [Member] | Options Held [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	0		0
Notional Disclosures [Abstract]
Notional	0		0
Derivative Financial Instruments, Liabilities [Member] | Total Return Swap [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	0		(17,000)
Notional Disclosures [Abstract]
Notional	$ 0		$ 90,592,668