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Leases - Additional Information (Detail)
$ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
Aircraft
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Disclosure of finance lease and operating lease by lessee [abstract]      
Minimum lease term of finance leases arrangements 10 years    
Acquisition of new aircraft as non-cash investing and financing transactions $ 89.0 $ 46.0  
Total lease expense $ 134.5 138.8 $ 142.2
Average term of aircraft operating leases 10 years    
Number of aircraft under lessor | Aircraft 2    
Carrying amount of aircraft under operating lease $ 37.0 41.6  
Scheduled expiration date of lease 2020    
Total lease income amounts $ 3.5 $ 3.5 $ 1.9