Investments - Additional Information (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) engine aircraft.	Sep. 30, 2018 USD ($) engine aircraft.	Dec. 31, 2018 USD ($)
Schedule of Equity Method Investments [Line Items]
Net book value of equipment held for operating lease	$ 1,624,937		$ 1,624,937		$ 1,673,135
Management fees included in other revenue	$ 3,105	$ 2,010	$ 10,674	$ 5,762
WMES
Schedule of Equity Method Investments [Line Items]
Ownership interest	50.00%		50.00%
Number of engines in lease portfolio | engine			32
Number of aircraft in lease portfolio | aircraft.			6
Net book value of equipment held for operating lease	$ 299,300		$ 299,300
Number of aircrafts sold | aircraft.			5
Proceeds from sale of aircraft			$ 76,400
Number of engines sold | engine				2
Proceeds from sale of engines				$ 23,200
WMES | Asset Management
Schedule of Equity Method Investments [Line Items]
Management fees included in other revenue	$ 700	$ 600	$ 1,800
Willis Aeronautical Services, Inc.
Schedule of Equity Method Investments [Line Items]
Number of engines sold | engine			1	1
Proceeds from sale of engines			$ 2,600	$ 2,600
CASC Willis
Schedule of Equity Method Investments [Line Items]
Ownership interest	50.00%		50.00%
Number of engines in lease portfolio | engine			4
Net book value of equipment held for operating lease	$ 48,700		$ 48,700
Number of aircrafts sold | aircraft.			0	0
Number of engines sold | engine			0	0