Debt Obligations - Schedule of Notes Payable (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2025	Dec. 31, 2024	Jun. 30, 2025
Long Term Debt
Variable rate (as a percent)	4.24%	4.37%
Gross amount of debt	$ 2,267,951,000	$ 2,292,726,000
Less: unamortized debt issuance costs and note discounts	(28,500,000)	(28,174,000)
Total debt obligations	$ 2,239,451,000	2,264,552,000
Credit facility at a floating rate of interest of one-month term Secured Overnight Financing Rate (“SOFR”) plus 2.35% at September 30, 2025, secured by engines, airframes, and loan assets. The credit facility has a committed amount of $1.0 billion at September 30, 2025, which revolves until the maturity date of October 2029.
Long Term Debt
Variable rate (as a percent)	2.35%
Line of credit facility outstanding amount	$ 497,000,000	693,000,000
WEST VIII Series A 2025 term notes payable at a fixed rate of interest of 5.58%, maturing in June 2050, secured by engines, airframes, and loan assets
Long Term Debt
Fixed rate (as a percent)	5.58%
Gross amount of debt	$ 519,492,000	0	$ 524,000,000.0
WEST VIII Series B 2025 term note payable at a fixed rate of interest of 6.07%, maturing in June 2050, secured by engines, airframes, and loan assets
Long Term Debt
Fixed rate (as a percent)	6.07%
Gross amount of debt	$ 71,381,000	0	$ 72,000,000.0
WEST VII Series A 2023 term notes payable at a fixed rate of interest of 8.00%, maturing in October 2048, secured by engines, airframes, and loan assets
Long Term Debt
Fixed rate (as a percent)	8.00%
Gross amount of debt	$ 237,500,000	356,355,000
WEST VI Series A 2021 term notes payable at a fixed rate of interest of 3.10%, maturing in May 2046, secured by engines, airframes, and loan assets
Long Term Debt
Fixed rate (as a percent)	3.10%
Gross amount of debt	$ 229,343,000	241,065,000
WEST VI Series B 2021 term notes payable at a fixed rate of interest of 5.44%, maturing in May 2046, secured by engines, airframes, and loan assets
Long Term Debt
Fixed rate (as a percent)	5.44%
Gross amount of debt	$ 31,839,000	33,486,000
WEST VI Series C 2021 term notes payable at a fixed rate of interest of 7.39%, maturing in May 2046, secured by engines, airframes, and loan assets
Long Term Debt
Fixed rate (as a percent)	7.39%
Gross amount of debt	$ 8,039,000	9,926,000
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	$ 216,870,000	226,572,000
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	$ 30,211,000	31,563,000
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	$ 6,255,000	8,142,000
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	$ 35,494,000	199,846,000
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	$ 9,868,000	27,338,000
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	$ 146,047,000	161,308,000
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	$ 19,609,000	21,659,000
Willis Warehouse Facility LLC (“WWFL”) credit facility at a floating rate of interest of one-month term SOFR, plus 1.88% at September 30, 2025 maturing in May 2030, secured by engines, airframes, and loan assets. The WWFL credit facility has a committed amount of $500.0 million at September 30, 2025.
Long Term Debt
Line of credit facility, maximum borrowing capacity	$ 500,000,000.0
Fixed rate (as a percent)	1.88%
Gross amount of debt	$ 84,788,000	221,882,000
Note payable at a fixed rate of interest of 5.00%, maturing in February 2033, secured by an engine
Long Term Debt
Fixed rate (as a percent)	5.00%
Gross amount of debt	$ 20,573,000	20,780,000
Note payable at a fixed rate of interest of 4.59%, maturing in November 2032, secured by an engine
Long Term Debt
Fixed rate (as a percent)	4.59%
Gross amount of debt	$ 21,692,000	22,094,000
Note payable at a fixed rate of interest of 4.23%, maturing in June 2032, secured by an engine
Long Term Debt
Fixed rate (as a percent)	4.23%
Gross amount of debt	$ 17,637,000	17,710,000
Note payable at a fixed rate of interest of 5.17%, maturing in March 2033, secured by an engine
Long Term Debt
Fixed rate (as a percent)	5.17%
Gross amount of debt	$ 23,718,000	0
Note payable at a fixed rate of interest of 5.91%, maturing in March 2034, secured by an engine
Long Term Debt
Fixed rate (as a percent)	5.91%
Gross amount of debt	$ 20,902,000	0
Note payable at a fixed rate of interest of 5.83%, maturing in April 2034, secured by an engine
Long Term Debt
Fixed rate (as a percent)	5.83%
Gross amount of debt	$ 19,693,000	$ 0
Revolving credit facility | Credit facility at a floating rate of interest of one-month term Secured Overnight Financing Rate (“SOFR”) plus 2.35% at September 30, 2025, secured by engines, airframes, and loan assets. The credit facility has a committed amount of $1.0 billion at September 30, 2025, which revolves until the maturity date of October 2029.
Long Term Debt
Line of credit facility, maximum borrowing capacity	$ 1,000,000,000.0