Schedule of Agency Securities in Unrealized Loss Position (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014 Securities	Dec. 31, 2013 Securities
Investments Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	$ 7,266,120	$ 7,973,580
Unrealized Loss - Less than 12 months	(91,717)	(120,019)
Number of securities in an unrealized loss position	233	279
Fannie Mae Certificates, ARMs
Investments Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	3,133,610	3,233,274
Unrealized Loss - Less than 12 months	(36,442)	(44,871)
Fannie Mae Certificates, Fixed Rate
Investments Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	171,707	281,760
Unrealized Loss - Less than 12 months	(345)	(1,798)
Freddie Mac Certificates, ARMs
Investments Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	3,806,999	4,046,473
Unrealized Loss - Less than 12 months	(54,734)	(70,752)
Freddie Mac Certificates, Fixed Rate
Investments Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months	153,804	316,835
Unrealized Loss - Less than 12 months	(196)	(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 Rate
Investments Unrealized Loss Position [Line Items]
Fair Market Value - Less than 12 months		95,238
Unrealized Loss - Less than 12 months		$ (998)