v3.8.0.1

INTANGIBLE ASSETS | 12 Months Ended
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Dec. 31, 2017
Intangible Assets, Net (Excluding Goodwill) [Abstract] |
INTANGIBLE ASSETS | INTANGIBLE ASSETS Intangible assets consisted of the following:
|  |  | December 31, 2017 |  | December 31, 2016
| Weighted-Average Amortization Period |  | Cost |  | Accumulated Amortization |  | Cost |  | Accumulated Amortization
| (Years) |  | (Thousands of Dollars)
Customer contracts and relationships | 18 |  | $ | 863,950 |  |  | $ | (81,136 | ) |  | $ | 166,950 |  |  | $ | (41,582 | )
Other | 47 |  | 2,359 |  |  | (694 | ) |  | 2,359 |  |  | (644 | )
Total |  |  | $ | 866,309 |  |  | $ | (81,830 | ) |  | $ | 169,309 |  |  | $ | (42,226 | )
All of our intangible assets are amortized on a straight-line basis. Amortization expense for intangible assets was $39.6 million, $13.9 million and $16.7 million for the years ended December 31, 2017, 2016 and 2015, respectively. The estimated aggregate amortization expense is approximately $51.0 million for each of the years 2018 through 2022.

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- Definition The entire disclosure for all or part of the information related to intangible assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -URI http://asc.fasb.org/topic&trid=2144416 + Details
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