Organization and Summary of Significant Accounting Policies (Organization and Revenue Recognition) (Details) $ in Thousands		12 Months Ended
	Aug. 04, 2017 USD ($) engine item	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) engine
Organizations [Line Items]
Number of engines | engine			393
Total		$ 1,103,743	$ 913,703
Number of business days to pay interest	5 days
Maturity term	10 years
Maintenance Reserves		75,889	71,602
Security Deposit Liability		25,302	21,417
Revenue Recognition
Lease rent to be received, more than 30 days past due		8,300
Maintenance reserve payments to be received, more than 30 days past due		$ 5,600
Minimum number of days for which lease rent and maintenance reserve payments are past due		30 days
Other Revenue, Net		$ 1,300	$ 4,000
Credit facility at a floating rate of interest of LIBOR plus 2.75%, secured by engines
Organizations [Line Items]
Total		491,000
WEST III Series A 2017-1 term notes payable at a fixed rate of interest of 4.69%, maturing in August 2042. Secured by engines
Organizations [Line Items]
Total		289,295
WEST III Series B 2017-1 term notes payable at a fixed rate of interest of 6.36%, maturing in August 2042. Secured by engines
Organizations [Line Items]
Total		41,370
WEST III
Organizations [Line Items]
Number of series notes | item	2
Maintenance Reserves		10,000
Security Deposit Liability		$ 1,000
WEST III | Revolving credit facility
Organizations [Line Items]
Number of engines | engine	56
WEST III | WEST III Series A 2017-1 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	$ 293,700
Fixed rate (as a percent)		4.69%
WEST III | WEST III Series B 2017-1 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	$ 42,000
Fixed rate (as a percent)		6.36%
WOLF
Organizations [Line Items]
Previously held interest (as a percent)		50.00%
Acquisition of the remaining outstanding shares (as a percent)		100.00%