Debt Obligations - Narrative (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) engine	Dec. 31, 2018 USD ($)	Jul. 31, 2019	Jun. 30, 2019 USD ($)	Feb. 28, 2019 USD ($)	Apr. 30, 2016 USD ($)
Long Term Debt [Line Items]
One month LIBOR at period end	1.76%	2.50%
Three month LIBOR at period end	1.91%
Loss on extinguishment of debt	$ 220,000	$ 0
Long-term debt outstanding	$ 1,270,469,000	1,358,430,000
Number of assets used to secure debt | engine	2
WEST IV
Long Term Debt [Line Items]
Long-term debt outstanding	$ 350,900,000	369,200,000
WEST III
Long Term Debt [Line Items]
Long-term debt outstanding	294,600,000	313,400,000
WEST II
Long Term Debt [Line Items]
Long-term debt outstanding	$ 211,572,000	237,847,000
Fixed rate (as a percent)	5.50%
Note payable at fixed interest rates ranging from 2.60% to 2.97%, maturing in July 2024, secured by an aircraft
Long Term Debt [Line Items]
Long-term debt outstanding	$ 9,124,000	10,937,000
Fixed rate (as a percent)	3.18%
Note payable due July 2022
Long Term Debt [Line Items]
Long-term debt outstanding	$ 7,286,000	0			$ 8,100,000
Maximum | Note payable at fixed interest rates ranging from 2.60% to 2.97%, maturing in July 2024, secured by an aircraft
Long Term Debt [Line Items]
Fixed rate (as a percent)			3.18%
LIBOR | Revolving credit facility
Long Term Debt [Line Items]
Variable rate spread	1.375%
LIBOR | Minimum | Note payable due July 2022
Long Term Debt [Line Items]
Variable rate spread	1.85%
LIBOR | Maximum | Note payable due July 2022
Long Term Debt [Line Items]
Variable rate spread	2.50%
Revolving credit facility | Revolving credit facility
Long Term Debt [Line Items]
Line of credit facility maximum borrowing Capacity before amendment						$ 890,000,000.0
Line of credit facility, maximum borrowing capacity under accordion feature	$ 1,300,000,000					$ 1,000,000,000.0
Line of credit facility, maximum borrowing capacity	1,000,000,000			$ 1,000,000,000.0
Debt issuance costs, line of credit				$ 2,800,000
Loss on extinguishment of debt	200,000
Maximum leverage ratio				4.50		4.00
Line of credit facility, remaining borrowing capacity	$ 603,000,000.0	$ 463,000,000.0