Leases, User Charges and Commitments - Future Payments Due (Detail) $ in Thousands	Dec. 31, 2017 USD ($)
Leases [Abstract]
Future Payments Due, Capital Lease, 2018	$ 3,137
Future Payments Due, Capital Lease, 2019	3,137
Future Payments Due, Capital Lease, 2020	3,145
Future Payments Due, Capital Lease, 2021	1,821
Future Payments Due, Capital Lease, Total	11,240
Less: Imputed interest	(767)
Net capital lease liability	10,473
Future Payments Due, Operating Leases and Other Commitments, 2018	10,755
Future Payments Due, Operating Leases and Other Commitments, 2019	8,622
Future Payments Due, Operating Leases and Other Commitments, 2020	7,700
Future Payments Due, Operating Leases and Other Commitments, 2021	7,272
Future Payments Due, Operating Leases and Other Commitments, 2022	6,576
Future Payments Due, Operating Leases and Other Commitments, 2023 and thereafter	7,494
Future Payments Due, Operating Leases and Other Commitments, Total	48,419
Future Payments Due, Total Leases, 2018	13,892
Future Payments Due, Total Leases, 2019	11,759
Future Payments Due, Total Leases, 2020	10,845
Future Payments Due, Total Leases, 2021	9,093
Future Payments Due, Total Leases, 2022	6,576
Future Payments Due, Total Leases, 2023 and thereafter	7,494
Future Payments Due, Leases, Total	$ 59,659