Schedule of MBS in Unrealized Loss Position (Details) $ in Thousands	Dec. 31, 2015 USD ($) Security	Dec. 31, 2014 USD ($) Security
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months	$ 4,524,126	$ 1,305,594
Unrealized Loss - Less than 12 months	(25,316)	(2,032)
Fair Market Value - Greater than 12 Months	1,373,082	3,043,913
Unrealized Loss - Greater than 12 Months	(19,848)	(33,499)
Total - Fair Market Value	5,897,208	4,349,507
Total - Unrealized Loss	$ (45,164)	$ (35,531)
Number of securities in an unrealized loss position, Less than 12 Months | Security	223	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	275	164
Fannie Mae Certificates | Adjustable Rate Residential Mortgage
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months	$ 1,712,043	$ 806,600
Unrealized Loss - Less than 12 months	(7,922)	(1,240)
Fair Market Value - Greater than 12 Months	511,884	1,306,153
Unrealized Loss - Greater than 12 Months	(6,542)	(12,497)
Total - Fair Market Value	2,223,927	2,112,753
Total - Unrealized Loss	(14,464)	(13,737)
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	1,814,773	498,994
Unrealized Loss - Less than 12 months	(9,246)	(792)
Fair Market Value - Greater than 12 Months	861,198	1,737,760
Unrealized Loss - Greater than 12 Months	(13,306)	(21,002)
Total - Fair Market Value	2,675,971	2,236,754
Total - Unrealized Loss	$ (22,552)	$ (21,794)