Derivative Instruments and Hedging Activities Put and Call Options for TBAs (Details) - USD ($)	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Derivative [Line Items]
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
Derivative Financial Instruments, Liabilities [Member]
Derivative [Line Items]
Notional	6,800,429,000		3,543,593,000
Fair Value	(11,312,000)		(12,501,000)
Derivative Financial Instruments, Assets [Member]
Derivative [Line Items]
Notional	20,122,892,000		21,708,907,000
Fair Value	238,305,000		324,182,000
Options Held [Member] | Derivative Financial Instruments, Liabilities [Member]
Derivative [Line Items]
Notional	0		0
Fair Value	0		0
Options Held [Member] | Derivative Financial Instruments, Assets [Member]
Derivative [Line Items]
Notional	2,000,000,000		1,136,000,000
Fair Value	156,000		42,633,000
Options Held [Member] | Long [Member]
Derivative [Line Items]
Notional	$ 2,000,000,000		2,500,000,000
Options Held [Member] | Short [Member]
Derivative [Line Items]
Notional			$ 3,636,000,000