Debt Obligations (Details) $ in Thousands	Aug. 22, 2018 USD ($) engine NotesSeries aircraft	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Principal outstanding repayable
2018		$ 13,812
2019		55,380
2020		54,980
2021		521,217
2022		207,733
Thereafter		560,261
Total		1,413,383	$ 1,103,743
Less: unamortized debt issuance costs		(21,270)	(18,338)
Total debt obligations		1,392,113	1,085,405
Credit facility at a floating rate of interest of one-month LIBOR plus 2.0% and maturity date of April 2021
Long Term Debt
Maximum borrowing capacity under credit facility		$ 890,000
Fixed rate (as a percent)		2.00%
Principal outstanding repayable
Total		$ 466,000	491,000
WEST IV
Long Term Debt
Number of series notes | NotesSeries	2
Aggregate principal amount	$ 373,400
Principal outstanding repayable
Total		$ 373,400
Number of engines pledged as collateral | engine	55
Number of airframes provided as collateral for debt | aircraft	1
WEST IV Series A 2018 term notes payable at a fixed rate of interest of 4.75%, maturing in September 2043, secured by engines
Long Term Debt
Fixed rate (as a percent)		4.75%
Notes issues at price of par (as percentage)	99.99504%
Aggregate principal amount	$ 326,800
Maturity term	8 years
Principal outstanding repayable
Total		$ 326,759
WEST IV Series B 2018 term notes payable at a fixed rate of interest of 5.44%, maturing in September 2043, secured by engines
Long Term Debt
Fixed rate (as a percent)		5.44%
Notes issues at price of par (as percentage)	99.99853%
Aggregate principal amount	$ 46,700
Maturity term	8 years
Principal outstanding repayable
Total		$ 46,680
WEST III Series A 2017 term notes payable at a fixed rate of interest of 4.69%, maturing in August 2042, secured by engines
Long Term Debt
Fixed rate (as a percent)		4.69%
Principal outstanding repayable
Total		$ 279,382	289,295
WEST III Series B 2017 term notes payable at a fixed rate of interest of 6.36%, maturing in August 2042, secured by engines
Long Term Debt
Fixed rate (as a percent)		6.36%
Principal outstanding repayable
Total		$ 39,953	41,370
WEST II Series A 2012 term notes payable at a fixed rate of interest of 5.50%, maturing in September 2037, secured by engines
Long Term Debt
Fixed rate (as a percent)		5.50%
Line of credit facility outstanding amount		$ 173,800
Principal outstanding repayable
Total		243,221	259,022
Note payable at fixed interest rates ranging from 2.60% to 2.97%, maturing in July 2024, secured by an aircraft
Principal outstanding repayable
Total		$ 11,388	12,720
Note payable at fixed interest rates ranging from 2.60% to 2.97%, maturing in July 2024, secured by an aircraft | Minimum
Long Term Debt
Fixed rate (as a percent)		2.60%
Note payable at fixed interest rates ranging from 2.60% to 2.97%, maturing in July 2024, secured by an aircraft | Maximum
Long Term Debt
Fixed rate (as a percent)		2.97%
Note payable at a variable interest rate of one-month LIBOR plus 2.25%, matured in January 2018, secured by engines
Long Term Debt
Fixed rate (as a percent)		2.25%
Principal outstanding repayable
Total			$ 10,336
Revolving credit facility
Long Term Debt
Maximum borrowing capacity under credit facility		$ 890,000
Amount of debt available under accordion feature		1,000,000
Line of credit facility outstanding amount		$ 466,000