Borrowings Under Repurchase Agreements - Weighted Average Maturities and Interest Rates of Borrowings (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Borrowings Outstanding	$ 1,013,162	$ 918,421
Deferred financing costs	(7,875)	(7,353)
Line of Credit [Member]
Debt Instrument [Line Items]
Borrowings Outstanding	1,013,162	918,421
Deferred financing costs	$ (7,875)	$ (7,353)
Weighted Average Rate	3.01%	2.92%
Line of Credit [Member] \| London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Basis point	0.53%	0.43%
Line of Credit [Member] \| JP Morgan Chase and Goldman Sachs [Member]
Debt Instrument [Line Items]
Borrowings Outstanding	$ 690,882	$ 491,870
Line of Credit [Member] \| JP Morgan Chase and Goldman Sachs [Member] \| London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Basis point	2.33%	2.37%
Line of Credit [Member] \| JP Morgan Chase [Member]
Debt Instrument [Line Items]
Maximum Amount of Borrowings	$ 943,000	$ 600,000
Borrowings Outstanding	648,086	445,942
Line of Credit [Member] \| Deutsche Bank Repurchase Facility [Member]
Debt Instrument [Line Items]
Maximum Amount of Borrowings	300,000
Borrowings Outstanding	0
Line of Credit [Member] \| Goldman Sachs [Member]
Debt Instrument [Line Items]
Borrowings Outstanding	$ 42,796	$ 45,928
Line of Credit [Member] \| Goldman Sachs [Member] \| London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Basis point	3.50%	3.50%
Line of Credit [Member] \| UBS and Deutsche Bank [Member]
Debt Instrument [Line Items]
Borrowings Outstanding	$ 330,155	$ 433,904
Weighted Average Rate	3.31%	3.39%
Line of Credit [Member] \| UBS [Member]
Debt Instrument [Line Items]
Borrowings Outstanding	$ 133,899	$ 133,899
Weighted Average Rate	2.79%	2.79%
Line of Credit [Member] \| Deutsche Bank [Member]
Debt Instrument [Line Items]
Borrowings Outstanding	$ 196,256	$ 300,005
Weighted Average Rate	3.66%	3.69%
Minimum [Member] \| Line of Credit [Member] \| JP Morgan Chase [Member] \| London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Basis point	2.40%	2.25%
Minimum [Member] \| Line of Credit [Member] \| Deutsche Bank [Member] \| London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Basis point	1.80%
Maximum [Member] \| Line of Credit [Member] \| Deutsche Bank [Member] \| London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Basis point	2.32%
Asset Specific Financing [Member] \| Line of Credit [Member] \| JP Morgan Chase [Member]
Debt Instrument [Line Items]
Long-term line of credit	$ 143,000
General Facility [Member] \| Line of Credit [Member] \| JP Morgan Chase [Member]
Debt Instrument [Line Items]
Long-term line of credit	$ 800,000

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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- Definition

Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
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- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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- Definition

Weighted average interest rate of long-term debt outstanding.

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- Definition

Amount, after the effects of master netting arrangements, of funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

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-Name Accounting Standards Codification
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Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Repurchase Agreement Accounted for as a Collateralized Borrowing
-URI http://asc.fasb.org/extlink&oid=49203159

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