Schedule of MBS in Unrealized Loss Position (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015 Securities	Dec. 31, 2014 Securities
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months	$ 35,712	$ 1,305,594
Unrealized Loss - Less than 12 months	(145)	(2,032)
Fair Market Value - Greater than 12 Months	1,718,119	3,043,913
Unrealized Loss - Greater than 12 Months	(8,994)	(33,499)
Total - Fair Market Value	1,753,831	4,349,507
Total - Unrealized Loss	(9,139)	(35,531)
Number of securities in an unrealized loss position, Less than 12 Months	2	56
Number of securities in an unrealized loss position, Greater than 12 Months	56	108
Number of securities in an unrealized loss position, Total	58	164
Fannie Mae Certificates | Adjustable Rate Residential Mortgage
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months	25,695	806,600
Unrealized Loss - Less than 12 months	(139)	(1,240)
Fair Market Value - Greater than 12 Months	624,502	1,306,153
Unrealized Loss - Greater than 12 Months	(2,297)	(12,497)
Total - Fair Market Value	650,197	2,112,753
Total - Unrealized Loss	(2,436)	(13,737)
Freddie Mac Certificates | Adjustable Rate Residential Mortgage
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months	10,017	498,994
Unrealized Loss - Less than 12 months	(6)	(792)
Fair Market Value - Greater than 12 Months	1,093,617	1,737,760
Unrealized Loss - Greater than 12 Months	(6,697)	(21,002)
Total - Fair Market Value	1,103,634	2,236,754
Total - Unrealized Loss	$ (6,703)	$ (21,794)