Schedule of MBS in Unrealized Loss Position (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014 Securities	Dec. 31, 2013 Securities
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months	$ 2,221,676	$ 7,973,580
Unrealized Loss - Less than 12 months	(5,598)	(120,019)
Fair Market Value - Greater than 12 Months	3,475,234	0
Unrealized Loss - Greater than 12 Months	(46,737)	0
Total - Fair Market Value	5,696,910	7,973,580
Total - Unrealized Loss	(52,335)	(120,019)
Number of securities in an unrealized loss position, Less than 12 Months	70	279
Number of securities in an unrealized loss position, Greater than 12 Months	128	0
Number of securities in an unrealized loss position, Total	198	279
Fannie Mae Certificates, ARMs
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months	966,859	3,233,274
Unrealized Loss - Less than 12 months	(2,084)	(44,871)
Fair Market Value - Greater than 12 Months	1,467,791	0
Unrealized Loss - Greater than 12 Months	(17,920)	0
Total - Fair Market Value	2,434,650	3,233,274
Total - Unrealized Loss	(20,004)	(44,871)
Fannie Mae Certificates, Fixed Rate
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months	295,297	281,760
Unrealized Loss - Less than 12 months	(615)	(1,798)
Fair Market Value - Greater than 12 Months	0	0
Unrealized Loss - Greater than 12 Months	0	0
Total - Fair Market Value	295,297	281,760
Total - Unrealized Loss	(615)	(1,798)
Freddie Mac Certificates, ARMs
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months	875,078	4,046,473
Unrealized Loss - Less than 12 months	(2,419)	(70,752)
Fair Market Value - Greater than 12 Months	2,007,443	0
Unrealized Loss - Greater than 12 Months	(28,817)	0
Total - Fair Market Value	2,882,521	4,046,473
Total - Unrealized Loss	(31,236)	(70,752)
Freddie Mac Certificates, Fixed Rate
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months	8,692	316,835
Unrealized Loss - Less than 12 months	(30)	(1,600)
Fair Market Value - Greater than 12 Months	0	0
Unrealized Loss - Greater than 12 Months	0	0
Total - Fair Market Value	8,692	316,835
Total - Unrealized Loss	(30)	(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)
Non-Agency ARMs
Schedule Of Available For Sale Securities [Line Items]
Fair Market Value - Less than 12 months	75,750
Unrealized Loss - Less than 12 months	(450)
Fair Market Value - Greater than 12 Months	0
Unrealized Loss - Greater than 12 Months	0
Total - Fair Market Value	75,750
Total - Unrealized Loss	$ (450)