Secured Debt Agreements, Net - Credit Facilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Credit Facilities [Member]
Line of Credit Facility [Line Items]
Potential	$ 9,444,814	$ 9,210,325
Repurchase Borrowings Outstanding	9,093,821	8,870,897
Available	350,993	339,428
Collateral Assets	12,007,414	11,729,560
Deutsche Bank [Member]
Line of Credit Facility [Line Items]
Repurchase Borrowings Outstanding	860,500
Deutsche Bank [Member] \| Credit Facilities [Member]
Line of Credit Facility [Line Items]
Potential	1,865,813	1,839,698
Repurchase Borrowings Outstanding	1,865,813	1,839,698
Collateral Assets	2,357,198	2,325,047
Wells Fargo [Member]
Line of Credit Facility [Line Items]
Repurchase Borrowings Outstanding	128,200	474,400
Wells Fargo [Member] \| Credit Facilities [Member]
Line of Credit Facility [Line Items]
Potential	1,992,903	1,908,509
Repurchase Borrowings Outstanding	1,748,737	1,822,154
Available	244,166	86,355
Collateral Assets	2,556,230	2,514,513
JP Morgan [Member]
Line of Credit Facility [Line Items]
Repurchase Borrowings Outstanding		520,600
JP Morgan [Member] \| Credit Facilities [Member]
Line of Credit Facility [Line Items]
Potential	1,078,749	1,010,628
Repurchase Borrowings Outstanding	1,078,749	1,010,628
Collateral Assets	1,357,997	1,266,259
Barclays [Member] \| Credit Facilities [Member]
Line of Credit Facility [Line Items]
Potential	890,620	890,620
Repurchase Borrowings Outstanding	890,620	890,620
Collateral Assets	1,113,275	1,113,275
Citibank [Member] \| Credit Facilities [Member]
Line of Credit Facility [Line Items]
Potential	878,104	852,470
Repurchase Borrowings Outstanding	801,474	663,917
Available	76,630	188,553
Collateral Assets	1,114,229	1,076,085
Bank of America [Member] \| Credit Facilities [Member]
Line of Credit Facility [Line Items]
Potential	793,804	873,446
Repurchase Borrowings Outstanding	793,804	873,446
Collateral Assets	992,805	1,090,117
MetLife [Member] \| Credit Facilities [Member]
Line of Credit Facility [Line Items]
Potential	675,329	675,329
Repurchase Borrowings Outstanding	675,329	675,329
Collateral Assets	857,689	852,733
Morgan Stanley [Member] \| Credit Facilities [Member]
Line of Credit Facility [Line Items]
Potential	444,653	341,241
Repurchase Borrowings Outstanding	414,456	276,721
Available	30,197	64,520
Collateral Assets	589,931	457,496
Societe Generale [Member] \| Credit Facilities [Member]
Line of Credit Facility [Line Items]
Potential	321,182	321,182
Repurchase Borrowings Outstanding	321,182	321,182
Collateral Assets	410,165	404,048
Goldman Sachs [Member] \| Credit Facilities [Member]
Line of Credit Facility [Line Items]
Potential	237,149	230,140
Repurchase Borrowings Outstanding	237,149	230,140
Collateral Assets	314,526	295,368
Goldman Sachs - Multi. JV [Member] \| Credit Facilities [Member]
Line of Credit Facility [Line Items]
Potential	169,506	170,060
Repurchase Borrowings Outstanding	169,506	170,060
Collateral Assets	221,202	212,983
Bank of America - Multi. JV [Member] \| Credit Facilities [Member]
Line of Credit Facility [Line Items]
Potential	97,002	97,002
Repurchase Borrowings Outstanding	97,002	97,002
Collateral Assets	$ 122,167	$ 121,636

Amount of assets pledged to secure a debt instrument.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

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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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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
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- Definition

Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

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Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

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