Schedule of Derivative Instruments in Statement of Financial Position, Fair Value (Details) - USD ($)	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Notional Disclosures [Abstract]
Notional	$ 41,330,283,000	$ 28,618,861,000	$ 31,226,878,000	$ 24,113,321,000	$ 16,988,116,000	$ 18,802,500,000
Inverse Interest-Only Securities [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	67,665,000		91,827,000
Notional Disclosures [Abstract]
Notional	498,826,000		588,246,000
Total Return Swap [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	2,000		(106,000)
Notional Disclosures [Abstract]
Notional	49,691,000		63,507,000
Derivative Financial Instruments, Assets [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	403,231,000		309,918,000
Notional Disclosures [Abstract]
Notional	33,229,283,000		25,503,371,000
Derivative Financial Instruments, Assets [Member] | Inverse Interest-Only Securities [Member]
Notional Disclosures [Abstract]
Notional	498,826,000		588,246,000
Derivative Financial Instruments, Assets [Member] | Interest Rate Swap [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	238,116,000		206,773,000
Notional Disclosures [Abstract]
Notional	28,128,766,000		21,516,125,000
Derivative Financial Instruments, Assets [Member] | Interest Rate Swaption [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	24,912,000		10,405,000
Notional Disclosures [Abstract]
Notional	164,000,000		2,666,000,000
Derivative Financial Instruments, Assets [Member] | Interest Rate Cap [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	52,370,000
Notional Disclosures [Abstract]
Notional	2,500,000,000
Derivative Financial Instruments, Assets [Member] | Forward Contracts [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	15,212,000		913,000
Notional Disclosures [Abstract]
Notional	1,108,000,000		733,000,000
Derivative Financial Instruments, Assets [Member] | Options Held [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	4,954,000		0
Notional Disclosures [Abstract]
Notional	780,000,000
Derivative Financial Instruments, Assets [Member] | Total Return Swap [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	2,000		0
Notional Disclosures [Abstract]
Notional	49,691,000		0
Derivative Financial Instruments, Assets [Member] | Short US Treasury Securities [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	0
Notional Disclosures [Abstract]
Notional	0
Derivative Financial Instruments, Liabilities [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	(797,504,000)		(31,903,000)
Notional Disclosures [Abstract]
Notional	11,521,000,000		8,335,507,000
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	0		(29,867,000)
Notional Disclosures [Abstract]
Notional	2,375,000,000		6,966,000,000
Derivative Financial Instruments, Liabilities [Member] | Interest Rate Swaption [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	0		0
Notional Disclosures [Abstract]
Notional	0		0
Derivative Financial Instruments, Liabilities [Member] | Interest Rate Cap [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	0
Notional Disclosures [Abstract]
Notional	0
Derivative Financial Instruments, Liabilities [Member] | Forward Contracts [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	(43,314,000)		(1,930,000)
Notional Disclosures [Abstract]
Notional	8,216,000,000		1,306,000,000
Derivative Financial Instruments, Liabilities [Member] | Options Held [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	(1,329,000)		0
Notional Disclosures [Abstract]
Notional	130,000,000
Derivative Financial Instruments, Liabilities [Member] | Total Return Swap [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	0		(106,000)
Notional Disclosures [Abstract]
Notional	0		63,507,000
Derivative Financial Instruments, Liabilities [Member] | Short US Treasury Securities [Member]
Derivative, Fair Value, Net [Abstract]
Fair Value	$ (752,861,000)		$ 0