Leases and Rentals - Additional Information (Details) - USD ($) $ in Thousands |
12 Months Ended | |||
|---|---|---|---|---|
Feb. 18, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|
| Capital Leased Assets [Line Items] | ||||
| Operating leases, rental expenses | $ 3,500 | $ 6,000 | $ 5,000 | |
| Total minimum lease payments | 5,513 | |||
| Depreciation on capital leases | $ 100 | $ 300 | $ 500 | |
| Minimum | Capital lease obligations | ||||
| Capital Leased Assets [Line Items] | ||||
| Capital lease term | 24 months | |||
| Maximum | Capital lease obligations | ||||
| Capital Leased Assets [Line Items] | ||||
| Capital lease term | 60 months | |||
| Subsequent event | Corporate Headquarters, Denver, CO | ||||
| Capital Leased Assets [Line Items] | ||||
| Total minimum lease payments | $ 8,300 | |||
| Term of lease contract | 10 years | |||