Schedule of MBS in Unrealized Loss Position (Details) $ in Thousands	Mar. 31, 2016 USD ($) Security	Dec. 31, 2015 USD ($) Security
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months	$ 1,396,312	$ 4,524,126
Unrealized Loss - Less than 12 months	(3,852)	(25,316)
Fair Market Value - Greater than 12 Months	1,392,886	1,373,082
Unrealized Loss - Greater than 12 Months	(7,658)	(19,848)
Total - Fair Market Value	2,789,198	5,897,208
Total - Unrealized Loss	$ (11,510)	$ (45,164)
Number of securities in an unrealized loss position, Less than 12 Months | Security	84	223
Number of securities in an unrealized loss position, Greater than 12 Months | Security	57	52
Number of securities in an unrealized loss position, Total | Security	141	275
Fannie Mae Certificates | Adjustable Rate Residential Mortgage
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months	$ 775,829	$ 1,712,043
Unrealized Loss - Less than 12 months	(2,022)	(7,922)
Fair Market Value - Greater than 12 Months	503,197	511,884
Unrealized Loss - Greater than 12 Months	(2,447)	(6,542)
Total - Fair Market Value	1,279,026	2,223,927
Total - Unrealized Loss	(4,469)	(14,464)
Fannie Mae Certificates | Fixed-Rate
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months		997,310
Unrealized Loss - Less than 12 months		(8,148)
Total - Fair Market Value		997,310
Total - Unrealized Loss		(8,148)
Freddie Mac Certificates | Adjustable Rate Residential Mortgage
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months	620,483	1,814,773
Unrealized Loss - Less than 12 months	(1,830)	(9,246)
Fair Market Value - Greater than 12 Months	889,689	861,198
Unrealized Loss - Greater than 12 Months	(5,211)	(13,306)
Total - Fair Market Value	1,510,172	2,675,971
Total - Unrealized Loss	$ (7,041)	$ (22,552)