Schedule of Agency Securities in Unrealized Loss Position (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013 Securities	Dec. 31, 2012 Securities
Investments Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	$ 7,973,580	$ 191,580
Unrealized Loss - Less than 12 months	(120,019)	(121)
Number of securities in an unrealized loss position	279	4
Fannie Mae Certificates, ARMs
Investments Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	3,233,274	149,954
Unrealized Loss - Less than 12 months	(44,871)	(100)
Fannie Mae Certificates, Fixed Rate
Investments Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	281,760
Unrealized Loss - Less than 12 months	(1,798)
Freddie Mac Certificates, ARMs
Investments Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	4,046,473	41,625
Unrealized Loss - Less than 12 months	(70,752)	(21)
Freddie Mac Certificates, Fixed Rate
Investments Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	316,835
Unrealized Loss - Less than 12 months	(1,600)
Ginnie Mae Certificates, Arms
Investments Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months
Unrealized Loss - Less than 12 months
Ginnie Mae Certificates, Fixed Rates
Investments Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	95,238
Unrealized Loss - Less than 12 months	$ (998)