Fair Value of Financial Instruments (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restricted cash	$ 54,702	$ 62,457
Securities, held-to-maturity	145,780	146,352
Carrying Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Cash and cash equivalents	142,905	200,996
Restricted cash	54,702	62,457
Carrying Value [Member] \| Level 3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Securities, held-to-maturity	145,780	146,352
Convertible senior notes, net	(250,468)	(249,994)
Carrying Value [Member] \| Level 3 [Member] \| Commercial Mortgage Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial mortgage and Subordinate loans	1,955,816	1,641,856
Carrying Value [Member] \| Level 3 [Member] \| Subordinate Mortgage Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial mortgage and Subordinate loans	1,195,570	1,051,236
Borrowings under repurchase agreements	(1,487,235)	(1,146,566)
Estimated Fair Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Cash and cash equivalents	142,905	200,996
Restricted cash	54,702	62,457
Estimated Fair Value [Member] \| Level 3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Securities, held-to-maturity	146,026	146,489
Convertible senior notes, net	(274,262)	(268,124)
Estimated Fair Value [Member] \| Level 3 [Member] \| Commercial Mortgage Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial mortgage and Subordinate loans	1,953,073	1,648,896
Estimated Fair Value [Member] \| Level 3 [Member] \| Subordinate Mortgage Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial mortgage and Subordinate loans	1,199,865	1,060,882
Borrowings under repurchase agreements	(1,485,831)	(1,146,807)
Participating Mortgages [Member] \| Carrying Value [Member] \| Level 3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Participations sold	(84,647)	(84,979)
Participating Mortgages [Member] \| Estimated Fair Value [Member] \| Level 3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Participations sold	$ (84,811)	$ (85,072)

Participating Mortgage Loans, Fair Value Disclosure

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Cash
-URI http://asc.fasb.org/extlink&oid=6506951

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Cash Equivalents
-URI http://asc.fasb.org/extlink&oid=6507016

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- Definition

Fair value portion of borrowing which can be exchanged for a specified number of another security at the option of the issuer or the holder, for example, but not limited to, the entity's common stock.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Convertible Security
-URI http://asc.fasb.org/extlink&oid=6509036

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Fair value of investments in debt securities classified as held-to-maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

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- Definition

Fair value portion of contractual arrangement with a lender under which borrowings can be made up to a specific amount at any point in time.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Line-of-Credit Arrangement
-URI http://asc.fasb.org/extlink&oid=6517033

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- Definition

Fair value portion of mortgage loans held-for-sale.

+ References

No definition available.

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- Definition

The carrying amounts of cash and cash equivalent items which are restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against short-term borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits; however, time deposits and short-term certificates of deposit are not generally included in legally restricted deposits. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. For a classified balance sheet represents the current portion only (the noncurrent portion has a separate concept); there is a separate and distinct element for unclassified presentations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.1)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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