Schedule of MBS in Unrealized Loss Position (Details) $ in Thousands	Sep. 30, 2015 USD ($) Security	Dec. 31, 2014 USD ($) Security
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months	$ 384,749	$ 1,305,594
Unrealized Loss - Less than 12 months	(612)	(2,032)
Fair Market Value - Greater than 12 Months	1,450,384	3,043,913
Unrealized Loss - Greater than 12 Months	(8,267)	(33,499)
Total - Fair Market Value	1,835,133	4,349,507
Total - Unrealized Loss	$ (8,879)	$ (35,531)
Number of securities in an unrealized loss position, Less than 12 Months | Security	23	56
Number of securities in an unrealized loss position, Greater than 12 Months | Security	52	108
Number of securities in an unrealized loss position, Total | Security	75	164
Fannie Mae Certificates | Adjustable Rate Residential Mortgage
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months	$ 177,892	$ 806,600
Unrealized Loss - Less than 12 months	(329)	(1,240)
Fair Market Value - Greater than 12 Months	539,176	1,306,153
Unrealized Loss - Greater than 12 Months	(2,212)	(12,497)
Total - Fair Market Value	717,068	2,112,753
Total - Unrealized Loss	(2,541)	(13,737)
Freddie Mac Certificates | Adjustable Rate Residential Mortgage
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months	206,857	498,994
Unrealized Loss - Less than 12 months	(283)	(792)
Fair Market Value - Greater than 12 Months	911,208	1,737,760
Unrealized Loss - Greater than 12 Months	(6,055)	(21,002)
Total - Fair Market Value	1,118,065	2,236,754
Total - Unrealized Loss	$ (6,338)	$ (21,794)