Mortgage-Backed Securities - By Type (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	$ 1,676,621,000	$ 1,549,171,000
Total Pass Through Certificates [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	1,596,207,000	1,486,787,000
Total Strucutured Certificates [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	80,414,000	62,384,000
Hybrid Adjustable Rate Mortgages [Member] | Total Pass Through Certificates [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	69,619,000	70,400,000
Adjustable-rate Mortgages [Member] | Total Pass Through Certificates [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	3,755,000	3,794,000
Fixed-rate Mortgages [Member] | Total Pass Through Certificates [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	1,522,833,000	1,412,593,000
Interest Only Securities [Member] | Total Strucutured Certificates [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	65,232,000	46,611,000
Inverse Interest Only Securities [Member] | Total Strucutured Certificates [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	$ 15,182,000	$ 15,773,000