Mortgage-Backed Securities - By Type (Details) - USD ($)	Sep. 30, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	$ 2,117,609,000	$ 1,549,171,000
Total Pass Through Certificates [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	2,013,409,000	1,486,787,000
Total Strucutured Certificates [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	104,200,000	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	54,655,000	70,400,000
Adjustable-rate Mortgages [Member] | Total Pass Through Certificates [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	3,090,000	3,794,000
Fixed-rate Mortgages [Member] | Total Pass Through Certificates [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	1,955,664,000	1,412,593,000
Interest Only Securities [Member] | Total Strucutured Certificates [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	64,351,000	46,611,000
Inverse Interest Only Securities [Member] | Total Strucutured Certificates [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair Value	$ 39,849,000	$ 15,773,000