Summary of Commercial Mortgage Loans, Mezzanine Loans, Preferred Equity and Other loans by Geographic location (Detail)	1 Months Ended	12 Months Ended
	Apr. 30, 2016 Loan	Dec. 31, 2016 USD ($) Loan		Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan	1	134
Carrying Amount of Mortgage Loans		$ 1,294,066,000	[1]	$ 1,625,838,000	$ 1,393,132,000
Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		0.00%
Principal		$ 0
Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		14.50%
Principal		$ 45,939,000
Texas
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		24
Carrying Amount of Mortgage Loans		$ 236,249,000
Texas | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		5.00%
Principal		$ 238,000
Texas | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		14.50%
Principal		$ 34,350,000
Florida
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		14
Carrying Amount of Mortgage Loans		$ 186,409,000
Florida | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		4.60%
Principal		$ 142,000
Florida | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		12.00%
Principal		$ 27,040,000
Various States
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		3
Carrying Amount of Mortgage Loans		$ 110,574,000
Various States | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		4.40%
Principal		$ 299,000
Various States | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		7.50%
Principal		$ 45,939,000
California
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		16
Carrying Amount of Mortgage Loans		$ 148,901,000
California | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		0.00%
Principal		$ 1,486,000
California | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		7.30%
Principal		$ 24,100,000
Ohio
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		7
Carrying Amount of Mortgage Loans		$ 65,570,000
Ohio | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		5.50%
Principal		$ 1,590,000
Ohio | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		6.50%
Principal		$ 20,000,000
Pennsylvania
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		4
Carrying Amount of Mortgage Loans		$ 34,574,000
Pennsylvania | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		5.00%
Principal		$ 884,000
Pennsylvania | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		11.90%
Principal		$ 15,500,000
Colorado
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		2
Carrying Amount of Mortgage Loans		$ 23,315,000
Colorado | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		5.10%
Principal		$ 820,000
Colorado | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		5.80%
Principal		$ 21,009,000
New York
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		3
Carrying Amount of Mortgage Loans		$ 18,625,000
New York | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		5.90%
Principal		$ 4,000,000
New York | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		9.70%
Principal		$ 7,875,000
Wisconsin
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		12
Carrying Amount of Mortgage Loans		$ 33,347,000
Wisconsin | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		5.50%
Principal		$ 500,000
Wisconsin | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		11.00%
Principal		$ 13,000,000
Maryland
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		3
Carrying Amount of Mortgage Loans		$ 24,990,000
Maryland | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		5.50%
Principal		$ 4,040,000
Maryland | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		7.00%
Principal		$ 12,000,000
North Carolina
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		1
Carrying Amount of Mortgage Loans		$ 24,500,000
North Carolina | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		5.50%
Principal		$ 650,000
North Carolina | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		5.50%
Principal		$ 23,850,000
Georgia
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		6
Carrying Amount of Mortgage Loans		$ 57,384,000
Georgia | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		5.50%
Principal		$ 380,000
Georgia | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		12.50%
Principal		$ 30,804,000
Illinois
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		5
Carrying Amount of Mortgage Loans		$ 48,757,000
Illinois | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		5.30%
Principal		$ 5,400,000
Illinois | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		6.80%
Principal		$ 13,000,000
Alabama
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		5
Carrying Amount of Mortgage Loans		$ 46,464,000
Alabama | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		5.00%
Principal		$ 6,739,000
Alabama | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		6.50%
Principal		$ 13,600,000
Minnesota
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		1
Carrying Amount of Mortgage Loans		$ 19,995,000
Minnesota | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		4.40%
Principal		$ 3,995,000
Minnesota | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		5.50%
Principal		$ 16,000,000
Massachusetts
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		1
Carrying Amount of Mortgage Loans		$ 18,381,000
Massachusetts | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		7.20%
Principal		$ 18,500,000
Massachusetts | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		7.20%
Principal		$ 18,500,000
Tennessee
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		2
Carrying Amount of Mortgage Loans		$ 13,409,000
Tennessee | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		6.00%
Principal		$ 6,159,000
Tennessee | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		7.00%
Principal		$ 7,250,000
New Jersey
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		1
Carrying Amount of Mortgage Loans		$ 4,000,000
New Jersey | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		11.00%
Principal		$ 4,000,000
New Jersey | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		11.00%
Principal		$ 4,000,000
Arizona
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		3
Carrying Amount of Mortgage Loans		$ 19,980,000
Arizona | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		5.30%
Principal		$ 2,000,000
Arizona | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		12.10%
Principal		$ 11,750,000
Indiana
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		2
Carrying Amount of Mortgage Loans		$ 1,954,000
Indiana | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		10.00%
Principal		$ 900,000
Indiana | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		12.00%
Principal		$ 1,054,000
Connecticut
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		1
Carrying Amount of Mortgage Loans		$ 8,302,000
Connecticut | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		12.00%
Principal		$ 554,000
Connecticut | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		12.00%
Principal		$ 4,000,000
OKLAHOMA
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		3
Carrying Amount of Mortgage Loans		$ 18,338,000
OKLAHOMA | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		5.50%
Principal		$ 4,878,000
OKLAHOMA | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		10.00%
Principal		$ 7,860,000
Michigan
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		1
Carrying Amount of Mortgage Loans		$ 6,800,000
Michigan | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		5.80%
Principal		$ 6,800,000
Michigan | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		5.80%
Principal		$ 6,800,000
Virginia
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		5
Carrying Amount of Mortgage Loans		$ 45,906,000
Virginia | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		4.80%
Principal		$ 5,500,000
Virginia | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		6.50%
Principal		$ 16,956,000
SOUTH CAROLINA
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		1
Carrying Amount of Mortgage Loans		$ 9,000,000
SOUTH CAROLINA | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		5.50%
Principal		$ 9,000,000
SOUTH CAROLINA | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		5.50%
Principal		$ 9,000,000
Missouri
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		1
Carrying Amount of Mortgage Loans		$ 2,600,000
Missouri | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		9.10%
Principal		$ 2,600,000
Missouri | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		9.10%
Principal		$ 2,600,000
Washington
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		1
Carrying Amount of Mortgage Loans		$ 11,000,000
Washington | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		6.50%
Principal		$ 11,000,000
Washington | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		6.50%
Principal		$ 11,000,000
Delaware
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		1
Carrying Amount of Mortgage Loans		$ 658,000
Delaware | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		12.50%
Principal		$ 658,000
Delaware | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		12.50%
Principal		$ 658,000
District of Columbia
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		1
Carrying Amount of Mortgage Loans		$ 6,185,000
District of Columbia | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		6.80%
Principal		$ 6,185,000
District of Columbia | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		6.80%
Principal		$ 6,185,000
New Mexico
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		1
Carrying Amount of Mortgage Loans		$ 4,550,000
New Mexico | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		6.00%
Principal		$ 4,550,000
New Mexico | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		6.00%
Principal		$ 4,550,000
Nevada
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		3
Carrying Amount of Mortgage Loans		$ 43,350,000
Nevada | Minimum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		6.00%
Principal		$ 8,400,000
Nevada | Maximum
Mortgage Loans On Real Estate [Line Items]
Interest Rate		6.30%
Principal		$ 26,000,000

[1] The tax basis of the commercial mortgage loans, mezzanine loans and preferred equity interests approximates the carrying amount.