Mortgage-Backed Securities - By Type (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	$ 1,549,171,000	$ 351,223,000
Total Pass Through Certificates [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	1,486,787,000	326,975,000
Total Strucutured Certificates [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	62,384,000	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	70,400,000	76,118,000
Adjustable-rate Mortgages [Member] | Total Pass Through Certificates [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	3,794,000	5,334,000
Fixed-rate Mortgages [Member] | Total Pass Through Certificates [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	1,412,593,000	245,523,000
Interest Only Securities [Member] | Total Strucutured Certificates [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	46,611,000	19,206,000
Inverse Interest Only Securities [Member] | Total Strucutured Certificates [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	$ 15,773,000	$ 5,042,000