Debt - Components of Long-term Debt - B (Narrative) (Detail) - 5.41% Senior Secured Notes [Member] $ in Millions, $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($)	Dec. 31, 2018 CAD ($)	Dec. 31, 2017
Debt Instrument [Line Items]
Interest rate		5.41%	5.41%
Value of locomotive units used as collateral		$ 111
Final repayment of principal amount	$ 44

Contractual interest rate for funds borrowed, under the debt agreement.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant