Mortgage-Backed and Credit Risk Transfer Securities - Schedule of Fair Value of Available-for-sale Securities and Securities Accounted for under Fair Value Option by Asset Type (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	$ 11,003,683	$ 11,722,375
Securities under Fair Value Option	6,618,551	6,468,379
Marketable Securities	17,622,234	18,190,754
Agency RMBS
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	6,668,962	7,270,896
Securities under Fair Value Option	5,453,832	5,305,011
Marketable Securities	12,122,794	12,575,907
Agency-CMO
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	190,930	203,351
Securities under Fair Value Option	63,024	70,592
Marketable Securities	253,954	273,943
CMBS
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	2,339,249	2,376,413
Securities under Fair Value Option	850,032	840,004
Marketable Securities	3,189,281	3,216,417
Non-Agency RMBS
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	1,174,590	1,236,178
Securities under Fair Value Option	20,362	21,430
Marketable Securities	1,194,952	1,257,608
GSE CRT
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	629,952	635,537
Securities under Fair Value Option	231,301	231,342
Marketable Securities	861,253	866,879
15 year fixed-rate | Agency RMBS
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	2,484,470	2,842,440
Securities under Fair Value Option	125,082	132,342
Marketable Securities	2,609,552	2,974,782
30 year fixed-rate | Agency RMBS
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	2,333,288	2,467,871
Securities under Fair Value Option	5,328,750	5,172,669
Marketable Securities	7,662,038	7,640,540
ARM | Agency RMBS
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	227,666	241,200
Securities under Fair Value Option	0	0
Marketable Securities	227,666	241,200
Hybrid ARM | Agency RMBS
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	1,623,538	1,719,385
Securities under Fair Value Option	0	0
Marketable Securities	$ 1,623,538	$ 1,719,385