Derivative Instruments and Hedging Activities Put and Call Options for TBAs (Details) - USD ($)	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
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
Derivative Financial Instruments, Assets [Member]
Derivative [Line Items]
Notional	33,229,283,000		25,503,371,000
Fair Value	403,231,000		309,918,000
Derivative Financial Instruments, Liabilities [Member]
Derivative [Line Items]
Notional	11,521,000,000		8,335,507,000
Fair Value	(797,504,000)		(31,903,000)
Options Held [Member] | Derivative Financial Instruments, Assets [Member]
Derivative [Line Items]
Notional	780,000,000
Fair Value	4,954,000		0
Options Held [Member] | Derivative Financial Instruments, Liabilities [Member]
Derivative [Line Items]
Notional	130,000,000
Fair Value	(1,329,000)		0
Options Held [Member] | Long [Member]
Derivative [Line Items]
Notional	3,700,000,000		0
Options Held [Member] | Short [Member]
Derivative [Line Items]
Notional	$ 4,600,000,000		$ 0