Mortgage-Backed Securities - By Type (Details) (USD $)	Mar. 31, 2014	Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	$ 747,757,500	$ 351,222,512
Total Pass Through Certificates [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	701,476,000	326,975,000
Total Strucutured Certificates [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	46,281,000	24,248,000
Hybrid Adjustable Rate Mortgages [Member] | Total Pass Through Certificates [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	75,850,000	76,118,000
Adjustable-rate Mortgages [Member] | Total Pass Through Certificates [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	4,698,000	5,334,000
Fixed-rate Mortgages [Member] | Total Pass Through Certificates [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	620,928,000	245,523,000
Interest Only Securities [Member] | Total Strucutured Certificates [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	35,681,000	19,206,000
Inverse Interest Only Securities [Member] | Total Strucutured Certificates [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	$ 10,600,000	$ 5,042,000