Nature of Operations and Significant Accounting Policies - Narrative (Details) mi in Thousands, $ in Millions	Jun. 30, 2018 state mi	Mar. 31, 2018 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total number of rail route miles | mi	21
Number of states rail network serves | state	23
Accumulated Other Comprehensive Income (Loss)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification adjustment		$ (107)
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification adjustment		$ 107