Related Party Transactions (Details)	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Related Party Transactions
Management fees	$ 1,941,000	$ 1,853,000	$ 1,830,000	$ 2,476,000	$ 2,503,000	$ 2,604,000	$ 2,588,000	$ 2,753,000
Estimated Fair Value
Related Party Transactions
Securitized debt, Fair value	10,945,000				10,659,000				$ 10,945,000	$ 10,659,000
Western Asset Management Company
Related Party Transactions
Management fees (as a percent)									1.50%
Renewal term of management agreement									1 year
Notice period to terminate the Management Agreement following initial term									180 days
Multiple of average annual management fees used to calculate termination fee | item									3
Prior period over which management fees were incurred used to calculate the termination fee under the Management Agreement									24 months
Notice period to terminate the Management Agreement for cause									30 days
Management fees									$ 8,100,000	10,400,000	$ 10,900,000
Reimbursable employee costs									1,400,000	740,000	$ 1,200,000
Management fees incurred but not yet paid	1,900,000				2,500,000				1,900,000	2,500,000
Accounts payable, related parties	99,000				83,000				$ 99,000	83,000
Western Asset Management Company | Minimum
Related Party Transactions
Proportion of affirmative votes by the entity's independent directors to terminate the Management Agreement (as a percent)									67.00%
Proportion of votes required by the entity's independent directors for acceptance of reduction in management fees (as a percent)									67.00%
Securitized debt | VIE | Estimated Fair Value
Related Party Transactions
Securitized debt, Fair value	10,900,000				10,700,000				$ 10,900,000	10,700,000
Securitized commercial loan | Securitized debt | VIE
Related Party Transactions
Securitized debt	$ 10,932,000				$ 11,000,000				$ 10,932,000	$ 11,000,000