Commitments and Contingencies - Additional Information (Details) - Subsidiaries - Threatened Litigation	3 Months Ended
Mar. 31, 2017 USD ($) claim
Denver Transit
Loss Contingencies [Line Items]
Number of claims filed (claims) \| claim	0
Denver Transit, Delivery of Train Management System
Loss Contingencies [Line Items]
Damages alleged	$ 0
Denver Transit, Installation of Constant Wireless Crossings
Loss Contingencies [Line Items]
Damages alleged	$ 29,900,000

The value (monetary amount) of the award the plaintiff seeks in the legal matter.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14557-108349

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14435-108349

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration