Fair Value of Financial Instruments - Carrying Value and Estimated Fair Value of Financial Instruments (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Carrying Value
Financial Assets
Other assets	$ 74,250	$ 74,250
Total	258,505	266,058
Financial Liabilities
Repurchase agreements	13,911,137	14,080,801
Secured loans	1,650,000	1,650,000
Exchangeable senior notes	0	143,231
Total	15,561,137	15,874,032
Carrying Value | Commercial loans, held-for-investment
Financial Assets
Commercial loans, held-for-investment	184,255	191,808
Estimated Fair Value
Financial Assets
Other assets	74,250	74,250
Total	258,760	266,180
Financial Liabilities
Repurchase agreements	13,910,631	14,080,460
Secured loans	1,650,000	1,650,000
Exchangeable senior notes	0	143,948
Total	15,560,631	15,874,408
Estimated Fair Value | Commercial loans, held-for-investment
Financial Assets
Commercial loans, held-for-investment	$ 184,510	$ 191,930