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Organization and Summary of Significant Accounting Policies - Organization and Revenue Recognition (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Aug. 22, 2018
Aug. 04, 2017
Revenue Recognition      
Lease rent receivables past due $ 2.7    
Maintenance reserve receivables past due $ 3.8    
Minimum number of days for which lease rent and maintenance reserve payments are past due 30 days    
Unbilled revenue associated with outstanding contracts $ 0.8    
WEST IV Series A 2018 term notes payable at a fixed rate of interest of 4.75%, maturing in September 2043, secured by engines      
Organizations [Line Items]      
Aggregate principal amount   $ 326.8  
WEST IV Series B 2018 term notes payable at a fixed rate of interest of 5.44%, maturing in September 2043, secured by engines      
Organizations [Line Items]      
Aggregate principal amount   $ 46.7  
WEST III | WEST III Series A 2017 term notes payable at a fixed rate of interest of 4.69%, maturing in August 2042, secured by engines      
Organizations [Line Items]      
Aggregate principal amount     $ 293.7
WEST III | WEST III Series B 2017 term notes payable at a fixed rate of interest of 6.36%, maturing in August 2042, secured by engines      
Organizations [Line Items]      
Aggregate principal amount     $ 42.0
WEST II      
Organizations [Line Items]      
Acquisition of the remaining outstanding shares (as a percent) 100.00%