Schedule of Agency Securities in Unrealized Loss Position (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014 Securities	Dec. 31, 2013 Securities
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months	$ 1,233,629	$ 7,973,580
Unrealized Loss - Less than 12 months	(6,072)	(120,019)
Fair Market Value - Greater than 12 Months	3,317,270
Unrealized Loss - Greater than 12 Months	(37,581)
Total - Fair Market Value	4,550,899	7,973,580
Total - Unrealized Loss	(43,653)	(120,019)
Number of securities in an unrealized loss position, Less than 12 Months	41	279
Number of securities in an unrealized loss position, Greater than 12 Months	114
Number of securities in an unrealized loss position, Total	155	279
Fannie Mae Certificates, ARMs
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months	814,169	3,233,274
Unrealized Loss - Less than 12 months	(3,757)	(44,871)
Fair Market Value - Greater than 12 Months	1,364,894
Unrealized Loss - Greater than 12 Months	(13,519)
Total - Fair Market Value	2,179,063	3,233,274
Total - Unrealized Loss	(17,276)	(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)
Fair Market Value - Greater than 12 Months		0
Unrealized Loss - Greater than 12 Months		0
Total - Fair Market Value		281,760
Total - Unrealized Loss		(1,798)
Freddie Mac Certificates, ARMs
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months	419,460	4,046,473
Unrealized Loss - Less than 12 months	(2,315)	(70,752)
Fair Market Value - Greater than 12 Months	1,952,376
Unrealized Loss - Greater than 12 Months	(24,062)
Total - Fair Market Value	2,371,836	4,046,473
Total - Unrealized Loss	(26,377)	(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)
Fair Market Value - Greater than 12 Months		0
Unrealized Loss - Greater than 12 Months		0
Total - Fair Market Value		316,835
Total - Unrealized Loss		(1,600)
Ginnie Mae Certificates Arms
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Greater than 12 Months		0
Unrealized Loss - Greater than 12 Months		0
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)
Fair Market Value - Greater than 12 Months		0
Unrealized Loss - Greater than 12 Months		0
Total - Fair Market Value		95,238
Total - Unrealized Loss		$ (998)