Schedule of Outstanding Debt (Parenthetical) (Detail)	3 Months Ended
	Mar. 31, 2018 USD ($)
Secured Equipment Notes due in 2024 [Member]
Debt Instrument [Line Items]
Equipment notes due period	2024
Payment Frequency	monthly
Secured Equipment Notes due in 2024 [Member] | Commencing on 2017 [Member]
Debt Instrument [Line Items]
Equipment notes, date of first required payment	2017
Secured Equipment Notes due in 2024 [Member] | Commencing on 2018 [Member]
Debt Instrument [Line Items]
Equipment notes, date of first required payment	2018
Secured Equipment Notes due in 2023 [Member]
Debt Instrument [Line Items]
Equipment notes due period	2023
Payment Frequency	monthly
Secured Equipment Notes due in 2023 [Member] | Commencing on 2017 [Member]
Debt Instrument [Line Items]
Equipment notes, date of first required payment	2017
Secured Equipment Notes due in 2023 [Member] | Commencing on 2018 [Member]
Debt Instrument [Line Items]
Equipment notes, date of first required payment	2018
Secured Equipment Notes due in 2023 [Member] | Commencing on 2016 [Member]
Debt Instrument [Line Items]
Equipment notes, date of first required payment	2016
Secured Equipment Notes due in 2022 [Member]
Debt Instrument [Line Items]
Equipment notes due period	2022
Payment Frequency	monthly
Secured Equipment Notes due in 2022 [Member] | Commencing on 2017 [Member]
Debt Instrument [Line Items]
Equipment notes, date of first required payment	2017
Secured Equipment Notes due in 2022 [Member] | Commencing on 2015 [Member]
Debt Instrument [Line Items]
Equipment notes, date of first required payment	2015
Secured Equipment Notes due in 2021 [Member]
Debt Instrument [Line Items]
Equipment notes due period	2021
Payment Frequency	monthly
Secured Equipment Notes due in 2021 [Member] | Commencing on 2017 [Member]
Debt Instrument [Line Items]
Equipment notes, date of first required payment	2017
Secured Equipment Notes due in 2021 [Member] | Commencing on 2014 [Member]
Debt Instrument [Line Items]
Equipment notes, date of first required payment	2014
Secured Equipment Notes due in 2020 [Member]
Debt Instrument [Line Items]
Equipment notes due period	2020
Payment Frequency	monthly
Secured Equipment Notes due in 2020 [Member] | Commencing on 2016 [Member]
Debt Instrument [Line Items]
Equipment notes, date of first required payment	2016
Secured Equipment Notes due in 2020 [Member] | Commencing on 2013 [Member]
Debt Instrument [Line Items]
Equipment notes, date of first required payment	2013
Secured Equipment Notes due in 2019 [Member]
Debt Instrument [Line Items]
Equipment notes due period	2019
Payment Frequency	monthly
Secured Equipment Notes due in 2019 [Member] | Commencing on 2015 [Member]
Debt Instrument [Line Items]
Equipment notes, date of first required payment	2015
Secured Equipment Notes due in 2019 [Member] | Commencing on 2013 [Member]
Debt Instrument [Line Items]
Equipment notes, date of first required payment	2013
Secured Equipment Notes due in 2018 [Member]
Debt Instrument [Line Items]
Equipment notes due period	2018
Payment Frequency	monthly
Secured Equipment Notes due in 2018 [Member] | Commencing on 2014 [Member]
Debt Instrument [Line Items]
Equipment notes, date of first required payment	2014
Secured Equipment Notes due in 2018 [Member] | Commencing on 2013 [Member]
Debt Instrument [Line Items]
Equipment notes, date of first required payment	2013
Minimum [Member] | Secured Equipment Notes due in 2024 [Member]
Debt Instrument [Line Items]
Monthly payments	$ 403
Interest rate secured debt	2.85%
Minimum [Member] | Secured Equipment Notes due in 2023 [Member]
Debt Instrument [Line Items]
Monthly payments	$ 669
Interest rate secured debt	2.23%
Minimum [Member] | Secured Equipment Notes due in 2022 [Member]
Debt Instrument [Line Items]
Monthly payments	$ 3,030
Interest rate secured debt	2.16%
Minimum [Member] | Secured Equipment Notes due in 2021 [Member]
Debt Instrument [Line Items]
Monthly payments	$ 1,940
Interest rate secured debt	2.04%
Minimum [Member] | Secured Equipment Notes due in 2020 [Member]
Debt Instrument [Line Items]
Monthly payments	$ 6,175
Interest rate secured debt	1.72%
Minimum [Member] | Secured Equipment Notes due in 2019 [Member]
Debt Instrument [Line Items]
Monthly payments	$ 1,594
Interest rate secured debt	1.87%
Minimum [Member] | Secured Equipment Notes due in 2018 [Member]
Debt Instrument [Line Items]
Monthly payments	$ 6,480
Interest rate secured debt	2.05%
Maximum [Member] | Secured Equipment Notes due in 2024 [Member]
Debt Instrument [Line Items]
Monthly payments	$ 83,000
Interest rate secured debt	3.41%
Maximum [Member] | Secured Equipment Notes due in 2023 [Member]
Debt Instrument [Line Items]
Monthly payments	$ 187,963
Interest rate secured debt	3.66%
Maximum [Member] | Secured Equipment Notes due in 2022 [Member]
Debt Instrument [Line Items]
Monthly payments	$ 254,190
Interest rate secured debt	2.87%
Maximum [Member] | Secured Equipment Notes due in 2021 [Member]
Debt Instrument [Line Items]
Monthly payments	$ 326,333
Interest rate secured debt	2.96%
Maximum [Member] | Secured Equipment Notes due in 2020 [Member]
Debt Instrument [Line Items]
Monthly payments	$ 398,496
Interest rate secured debt	2.78%
Maximum [Member] | Secured Equipment Notes due in 2019 [Member]
Debt Instrument [Line Items]
Monthly payments	$ 325,050
Interest rate secured debt	2.62%
Maximum [Member] | Secured Equipment Notes due in 2018 [Member]
Debt Instrument [Line Items]
Monthly payments	$ 163,428
Interest rate secured debt	2.70%