Operating Leases (Narrative) (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) AircraftEngine	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) AircraftEngine	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Lessee Lease Description [Line Items]
Operating lease costs	$ 22,100	$ 21,500	$ 65,600	$ 65,600
Number of aircraft engines | AircraftEngine	13		13
Net book value of aircraft engines	$ 1,043,322		$ 1,043,322		$ 372,939
Lease income	$ 5,200		$ 18,500
Operating Lease, Income, Comprehensive Income [Extensible List]	us-gaap:OtherIncomeMember		us-gaap:OtherIncomeMember
Aircraft Engines [Member]
Lessee Lease Description [Line Items]
Net book value of aircraft engines	$ 179,100		$ 179,100