Debt Obligations - Schedule of Notes Payable (Details) - USD ($)	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Long Term Debt
Gross amount of debt	$ 1,482,867,000	$ 1,270,469,000
Less: unamortized debt issuance costs	(20,137,000)	(19,463,000)
Total debt obligations	1,462,730,000	1,251,006,000
Credit facility at a floating rate of interest of one-month LIBOR plus 1.38% at September 30, 2020, secured by engines. The facility has a committed amount of $1.0 billion at September 30, 2020, which revolves until the maturity date of June 2024
Long Term Debt
Line of credit facility outstanding amount	$ 518,000,000	397,000,000
WEST V Series A 2020 term notes payable at a fixed rate of interest of 3.23%, maturing in March 2045, secured by engines
Long Term Debt
Fixed rate (as a percent)	3.23%
Gross amount of debt	$ 296,101,000	0
WEST V Series B 2020 term notes payable at a fixed rate of interest of 4.21%, maturing in March 2045, secured by engines
Long Term Debt
Fixed rate (as a percent)	4.21%
Gross amount of debt	$ 41,152,000	0
WEST V Series C 2020 term notes payable at a fixed rate of interest of 6.66%, maturing in March 2045, secured by engines
Long Term Debt
Fixed rate (as a percent)	6.66%
Gross amount of debt	$ 19,781,000	0
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%
Gross amount of debt	$ 283,651,000	307,014,000
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%
Gross amount of debt	$ 40,522,000	43,859,000
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%
Gross amount of debt	$ 235,790,000	257,754,000
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%
Gross amount of debt	$ 33,719,000	36,860,000
WEST II Series A 2012 term notes payable at a fixed rate of interest of 5.50%, repaid in March 2020, secured by engines
Long Term Debt
Fixed rate (as a percent)	5.50%
Gross amount of debt	$ 0	211,572,000
Note payable at three-month LIBOR plus a margin ranging from 1.85% to 2.50% at September 30, 2020, maturing in July 2022, secured by engines
Long Term Debt
Gross amount of debt	$ 6,429,000	7,286,000
Note payable at a fixed rate of interest of 3.18%, maturing in July 2024, secured by an aircraft
Long Term Debt
Fixed rate (as a percent)	3.18%
Gross amount of debt	$ 7,722,000	$ 9,124,000
Revolving credit facility | Credit facility at a floating rate of interest of one-month LIBOR plus 1.38% at September 30, 2020, secured by engines. The facility has a committed amount of $1.0 billion at September 30, 2020, which revolves until the maturity date of June 2024
Long Term Debt
Line of credit facility, maximum borrowing capacity			$ 1,000,000,000.0
LIBOR | Credit facility at a floating rate of interest of one-month LIBOR plus 1.38% at September 30, 2020, secured by engines. The facility has a committed amount of $1.0 billion at September 30, 2020, which revolves until the maturity date of June 2024
Long Term Debt
Variable rate (as a percent)	1.38%
LIBOR | Note payable at three-month LIBOR plus a margin ranging from 1.85% to 2.50% at September 30, 2020, maturing in July 2022, secured by engines | Minimum
Long Term Debt
Variable rate (as a percent)	1.85%
LIBOR | Note payable at three-month LIBOR plus a margin ranging from 1.85% to 2.50% at September 30, 2020, maturing in July 2022, secured by engines | Maximum
Long Term Debt
Variable rate (as a percent)	2.50%