Summary of Significant Accounting Policies (Detail)	12 Months Ended
	Dec. 31, 2019 USD ($) loan Subsidiary	Dec. 31, 2018 USD ($) loan	Dec. 31, 2017 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Foreign currency gain (losses)	$ 0	$ (1,038,000)	$ 4,103,000
FDIC deposit insurance limit amount	$ 250,000
Number of subsidiaries treated as taxable REIT | Subsidiary	2
MBS and GSE CRT Securities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities accounted for under fair value option	$ 17,366,360,000	$ 11,602,815,000
Percentage of MBS and GSE CRT accounted for under the fair value option	80.00%	67.00%
Available-for-sale securities	$ 4,405,426,000	$ 5,793,827,000
Percentage of MBS and GSE CRT securities classified as available-for-sale	20.00%	33.00%
Commercial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of loans | loan	1	2
Investment Denominated in Pound Sterling | Commercial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Foreign currency gain (losses)		$ 930,000	$ 4,100,000