Fair Value of Financial Instruments - Fair Value Measured on Recurring Basis (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Mortgage-backed and credit risk transfer securities, at fair value (including pledged securities of $17,185,870 and $17,560,811, respectively)	$ 17,622,234	$ 18,190,754
Derivative Assets	26,385	6,896
Derivative Liabilities	$ 8,204	$ 22,445
Weighted average remaining term of investments in unconsolidated ventures	2 years 4 months	1 year 11 months
Securities under Fair Value Option	$ 6,618,551	$ 6,468,379
GSE CRT
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Mortgage-backed and credit risk transfer securities, at fair value (including pledged securities of $17,185,870 and $17,560,811, respectively)	861,253	866,879
Securities under Fair Value Option	231,301	231,342
Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Mortgage-backed and credit risk transfer securities, at fair value (including pledged securities of $17,185,870 and $17,560,811, respectively)	17,622,234	18,190,754
Derivative Assets	26,385	6,896
Other assets, at fair value	32,134	29,951
NAV as a practical expedient	28,168	25,972
Total assets	17,680,753	18,227,601
Derivative Liabilities	20,354	32,765
Total liabilities	20,354	32,765
Recurring | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Mortgage-backed and credit risk transfer securities, at fair value (including pledged securities of $17,185,870 and $17,560,811, respectively)	0	0
Derivative Assets	0	0
Other assets, at fair value	3,966	3,979
Total assets	3,966	3,979
Derivative Liabilities	1,612	0
Total liabilities	1,612	0
Recurring | Level 1 | Exchange-traded funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Securities under Fair Value Option	4,000	4,000
Recurring | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Mortgage-backed and credit risk transfer securities, at fair value (including pledged securities of $17,185,870 and $17,560,811, respectively)	17,579,302	18,145,354
Derivative Assets	26,385	6,896
Other assets, at fair value	0	0
Total assets	17,605,687	18,152,250
Derivative Liabilities	18,742	32,765
Total liabilities	18,742	32,765
Recurring | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Mortgage-backed and credit risk transfer securities, at fair value (including pledged securities of $17,185,870 and $17,560,811, respectively)	42,932	45,400
Derivative Assets	0	0
Other assets, at fair value	0	0
Total assets	42,932	45,400
Derivative Liabilities	0	0
Total liabilities	0	0
Recurring | Level 3 | GSE CRT
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Embedded derivatives at fair value	42,900	45,400
Embedded derivatives in an asset position	44,100	46,500
Embedded derivatives in a liability position	$ (1,200)	$ 1,100