Notes Payable (Details) $ in Thousands			9 Months Ended
	Aug. 04, 2017 USD ($) item	Apr. 20, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Apr. 19, 2016 USD ($)
Long Term Debt
Number of business days to pay interest	5 days
Maturity term	10 years
Maintenance Reserves			$ 69,600		$ 71,602
Security Deposit Liability			24,706		21,417
Principal outstanding repayable
2017			9,722
2018			48,401
2019			38,537
2020			38,137
2021			367,074
Thereafter			449,295
Notes payable			951,166		913,703
Less: unamortized debt issuance costs			(19,034)		(13,448)
Total notes payable			932,132		$ 900,255
Net proceeds received from notes issued and sold			485,700	$ 87,000
WEST III
Long Term Debt
Number of series notes | item	2
Maintenance Reserves			5,000
Security Deposit Liability			$ 1,000
WEST II Series 2012-A term notes payable at a fixed rate of interest maturing in September 2037. Secured by engines | WEST II
Long Term Debt
Fixed rate (as a percent)			5.50%		5.50%
Principal outstanding repayable
Notes payable			$ 264,187		$ 279,541
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
Long Term Debt
Line of credit facility outstanding amount			$ 334,400
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 | WEST III
Long Term Debt
Fixed rate (as a percent)			4.69%
Aggregate principal amount	$ 293,700
Principal outstanding repayable
Notes payable			$ 292,599
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 | WEST III
Long Term Debt
Fixed rate (as a percent)			6.36%
Aggregate principal amount	$ 42,000
Principal outstanding repayable
Notes payable			$ 41,843
Note payable at fixed interest rates ranging from 2.60% to 2.97%, maturing in July 2024. Secured by an aircraft
Principal outstanding repayable
Notes payable			$ 13,158		14,453
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 2.25%, maturing in January 2018, secured by engines
Principal outstanding repayable
Notes payable			$ 10,679		11,709
Note payable at a variable interest rate of 2.25%, maturing in January 2018, secured by engines | LIBOR
Long Term Debt
Basis spread on variable rate (as a percent)			2.25%
Revolving credit facility
Long Term Debt
Basis spread on variable rate (as a percent)		2.75%
Maximum borrowing capacity under credit facility		$ 890,000				$ 700,000
Amount of debt available under accordion feature		$ 1,000,000
Revolving credit facility | WEST III
Long Term Debt
Number of Engines | item	56
Credit facility at a floating rate of interest of LIBOR plus 2.75%, secured by engines
Long Term Debt
Maximum borrowing capacity under credit facility			$ 890,000
Principal outstanding repayable
Notes payable			$ 328,700		$ 608,000
Credit facility at a floating rate of interest of LIBOR plus 2.75%, secured by engines | LIBOR
Long Term Debt
Basis spread on variable rate (as a percent)			2.75%