Fair Value of Financial Instruments - Carrying Value and Estimated Fair Value of Company's Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Securities, held-to-maturity	$ 0	$ 146,352
Carrying Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Cash and cash equivalents	77,671	200,996
Restricted cash	0	62,457
Carrying Value [Member] \| Level 3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Securities, held-to-maturity	0	146,352
Secured debt arrangements	(1,345,195)	(1,146,566)
Estimate of Fair Value Measurement [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Cash and cash equivalents	77,671	200,996
Restricted cash	0	62,457
Estimate of Fair Value Measurement [Member] \| Level 3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Securities, held-to-maturity	0	146,489
Secured debt arrangements	(1,345,195)	(1,146,807)
Commercial Mortgage Portfolio Segment [Member] \| Carrying Value [Member] \| Level 3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,653,826	1,641,856
Commercial Mortgage Portfolio Segment [Member] \| Estimate of Fair Value Measurement [Member] \| Level 3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,657,262	1,648,896
Subordinate Mortgage Portfolio Segment [Member] \| Carrying Value [Member] \| Level 3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,025,932	1,051,236
Subordinate Mortgage Portfolio Segment [Member] \| Estimate of Fair Value Measurement [Member] \| Level 3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,029,390	1,060,882
Participating Mortgages [Member] \| Carrying Value [Member] \| Level 3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Participations sold	0	(84,979)
Participating Mortgages [Member] \| Estimate of Fair Value Measurement [Member] \| Level 3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Participations sold	0	(85,072)
2019 Notes [Member] \| Carrying Value [Member] \| Level 3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
2019 Notes	(251,935)	(249,994)
2019 Notes [Member] \| Estimate of Fair Value Measurement [Member] \| Level 3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
2019 Notes	(276,506)	(268,124)
2022 Notes [Member] \| Carrying Value [Member] \| Level 3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
2019 Notes	(332,962)	0
2022 Notes [Member] \| Estimate of Fair Value Measurement [Member] \| Level 3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
2019 Notes	$ (350,175)	$ 0

Participating Mortgage Loans, Fair Value Disclosure

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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

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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.

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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
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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.

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- Definition

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

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Amount of cash and cash equivalents restricted as to withdrawal or usage, classified as current. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, 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.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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