Investments - Summarized Financial Information (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Income Statement [Abstract]
Revenue		$ 70,613	$ 120,366	$ 227,205	$ 319,932
Expenses		66,031	91,453	205,510	244,022
Net income		2,984	24,073	12,642	61,926
Statement of Financial Position [Abstract]
Total assets	[1]	2,149,223		2,149,223			$ 1,940,608
Total liabilities	[2]	1,735,227		1,735,227			1,540,632
Total WMES net equity		364,295	345,493	364,295	345,493	$ 357,868	350,338	$ 320,413	$ 286,787
WMES
Income Statement [Abstract]
Revenue		9,610	13,007	29,525	36,144
Expenses		7,539	10,076	25,718	28,436
Net income		2,071	$ 2,931	3,807	$ 7,708
Statement of Financial Position [Abstract]
Total assets		310,722		310,722			322,606
Total liabilities		225,761		225,761			227,052
Total WMES net equity		$ 84,961		$ 84,961			$ 95,554

[1] Total assets at September 30, 2020 and December 31, 2019, respectively, include the following assets of variable interest entities (“VIEs”) that can only be used to settle the liabilities of the VIEs: Cash nil and $134; Restricted cash $66,317 and $56,523; Equipment $1,081,757 and $1,004,851; Maintenance Rights $767 and $3,133; Inventory $4,367 and $2,832; and Other assets $1,240  and $668, respectively.

[2] Total liabilities at September 30, 2020 and December 31, 2019, respectively, include the following liabilities of VIEs for which the VIEs’ creditors do not have recourse to Willis Lease Finance Corporation: Debt obligations $935,064 and $842,996, respectively.