Schedule of MBS in Unrealized Loss Position (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014 Securities	Dec. 31, 2013 Securities
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months	$ 1,305,594	$ 7,973,580
Unrealized Loss - Less than 12 months	(2,032)	(120,019)
Fair Market Value - Greater than 12 Months	3,043,913
Unrealized Loss - Greater than 12 Months	(33,499)
Total - Fair Market Value	4,349,507	7,973,580
Total - Unrealized Loss	(35,531)	(120,019)
Number of securities in an unrealized loss position, Less than 12 Months	56	279
Number of securities in an unrealized loss position, Greater than 12 Months	108
Number of securities in an unrealized loss position, Total	164	279
Fannie Mae Certificates \| Adjustable Rate Residential Mortgage
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months	806,600	3,233,274
Unrealized Loss - Less than 12 months	(1,240)	(44,871)
Fair Market Value - Greater than 12 Months	1,306,153
Unrealized Loss - Greater than 12 Months	(12,497)
Total - Fair Market Value	2,112,753	3,233,274
Total - Unrealized Loss	(13,737)	(44,871)
Fannie Mae Certificates \| Fixed-Rate
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months		281,760
Unrealized Loss - Less than 12 months		(1,798)
Total - Fair Market Value		281,760
Total - Unrealized Loss		(1,798)
Freddie Mac Certificates \| Adjustable Rate Residential Mortgage
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months	498,994	4,046,473
Unrealized Loss - Less than 12 months	(792)	(70,752)
Fair Market Value - Greater than 12 Months	1,737,760
Unrealized Loss - Greater than 12 Months	(21,002)
Total - Fair Market Value	2,236,754	4,046,473
Total - Unrealized Loss	(21,794)	(70,752)
Freddie Mac Certificates \| Fixed-Rate
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months		316,835
Unrealized Loss - Less than 12 months		(1,600)
Total - Fair Market Value		316,835
Total - Unrealized Loss		(1,600)
Ginnie Mae Certificates \| Fixed-Rate
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months		95,238
Unrealized Loss - Less than 12 months		(998)
Total - Fair Market Value		95,238
Total - Unrealized Loss		$ (998)

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

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