Minimum amount to be expended to satisfy the terms of arrangements in which the entity has agreed to expend funds to procure goods or services, excluding long-term purchase commitments or unconditional purchase obligations.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(17))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(19))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: 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(25))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_PurchaseCommitmentRemainingMinimumAmountCommitted
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Leases, User Charges and Commitments - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2019
Lease and Rental Expense [Line Items]
Rental expense	$ 10,600,000	$ 9,500,000	$ 8,100,000
Rail transportation charges	80,400,000	77,600,000	73,700,000
Minimum commitments on rail owned chassis and containers	0
Expected annual subleases income	500,000
ASU No. 2016-02 [Member] \| Subsequent Event [Member]
Lease and Rental Expense [Line Items]
Right-of-use asset				$ 29,000,000
Lease liability				37,000,000
Estimated ROU assets and lease liabilities				$ 6,700
Transportation Costs [Member]
Lease and Rental Expense [Line Items]
Rental expense	$ 10,700,000	$ 6,000,000.0	$ 3,600,000