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Long-Term Debt - Additional Information (Details)					1 Months Ended	5 Months Ended	9 Months Ended
Long-Term Debt - Additional Information (Details)	Aug. 20, 2019	Sep. 14, 2018 USD ($)	Nov. 03, 2016 USD ($)	Jun. 22, 2016 USD ($)	Aug. 31, 2013 USD ($)	Nov. 25, 2016	Sep. 30, 2018 USD ($)	Jun. 08, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 21, 2017 USD ($)	Jun. 05, 2014 USD ($)	Mar. 05, 2014 USD ($) investor
Debt Instrument [Line Items]
Debt issued		$ 2,500,000,000.0
Weighted average interest rate (as a percent)							3.49%
Faiveley Transport
Debt Instrument [Line Items]
Acquired loans										$ 1,564,431,000
Forward Starting Interest Rate Swap Agreement
Debt Instrument [Line Items]
Interest rate swap agreements, notional amount											$ 150,000,000
Interest rate on notional value (as a percent)											2.56%
Floating Rate Senior Notes
Debt Instrument [Line Items]
Long-term debt							$ 496,495,000		$ 0
Senior Notes due 2024
Debt Instrument [Line Items]
Stated interest rate (as a percent)							4.15%
Long-term debt							$ 742,616,000		0
Senior Notes due 2028
Debt Instrument [Line Items]
Stated interest rate (as a percent)							4.70%
Long-term debt							$ 1,239,417,000		0
2018 Refinancing Agreement
Debt Instrument [Line Items]
Unrestricted cash							$ 300,000,000.0
EBITDA to interest expense ratio							3.00
Leverage ratio							3.25
Cash consideration transfer maximum							$ 500,000,000.0
Leverage ratio, three quarters following acquisition							3.75
Leverage ratio quarter four and five following acquisition							3.50
2018 Refinancing Agreement \| LIBOR/CDOR \| Minimum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate							1.00%
2018 Refinancing Agreement \| LIBOR/CDOR \| Maximum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate							1.875%
2018 Refinancing Agreement \| Base Rate \| Minimum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate							0.00%
2018 Refinancing Agreement \| Base Rate \| Maximum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate							0.875%
2018 Refinancing Agreement \| Letter of Credit
Debt Instrument [Line Items]
Line of credit amount outstanding							$ 29,700,000
Senior Notes Due 2026
Debt Instrument [Line Items]
Stated interest rate (as a percent)							3.45%
Long-term debt							$ 748,225,000		747,655,000
4.375% Senior Notes, due 2023
Debt Instrument [Line Items]
Stated interest rate (as a percent)							4.375%
Long-term debt							$ 248,759,000		248,567,000
2016 Refinancing Credit Agreement
Debt Instrument [Line Items]
Deferred financing cost				$ 3,300,000
2016 Refinancing Credit Agreement \| Federal Funds Effective Rate
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate							0.50%
2016 Refinancing Credit Agreement \| LIBOR Rate
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate							1.00%
2016 Refinancing Credit Agreement \| Base Rate
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate							0.00%
2016 Refinancing Credit Agreement \| Base Rate \| Minimum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate							0.00%
2016 Refinancing Credit Agreement \| Base Rate \| Maximum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate							0.75%
2016 Refinancing Credit Agreement \| Alternative Rate
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate							1.75%
2016 Refinancing Credit Agreement \| Alternative Rate \| Minimum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate							0.75%
2016 Refinancing Credit Agreement \| Alternative Rate \| Maximum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate							1.75%
2016 Refinancing Credit Agreement \| Term Loan
Debt Instrument [Line Items]
Ticking fee						0.10%
Schuldschein Loan
Debt Instrument [Line Items]
Long-term debt							$ 11,601,000		11,998,000
Schuldschein Loan \| Faiveley Transport
Debt Instrument [Line Items]
Stated interest rate (as a percent)							4.00%
Acquired loans												$ 137,200,000
Number of investors \| investor												20
Repayments of debt							$ 125,800,000
Long-term debt							$ 11,600,000
Senior Notes \| Minimum
Debt Instrument [Line Items]
Interest rate adjustment (as a percent)							0.25%
Senior Notes \| Maximum
Debt Instrument [Line Items]
Interest rate adjustment (as a percent)							1.00%
Senior Notes \| Floating Rate Senior Notes
Debt Instrument [Line Items]
Debt issued		$ 500,000,000
Debt instrument term		1 year
Debt issuance price (as a percent)		100.00%
Deferred financing cost		$ 3,500,000
Long-term debt							$ 496,495,000		0
Senior Notes \| Floating Rate Senior Notes \| LIBOR Rate
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		1.05%
Senior Notes \| Senior Notes due 2024
Debt Instrument [Line Items]
Debt issued		$ 750,000,000.0
Debt issuance price (as a percent)		99.805%
Stated interest rate (as a percent)		4.15%
Deferred financing cost		$ 7,400,000
Long-term debt							742,616,000		0
Senior Notes \| Senior Notes due 2028
Debt Instrument [Line Items]
Debt issued		$ 1,250,000,000.00
Debt issuance price (as a percent)		99.889%
Stated interest rate (as a percent)		4.70%
Deferred financing cost		$ 10,600,000
Long-term debt							1,239,417,000		0
Senior Notes \| Senior Notes Due 2026
Debt Instrument [Line Items]
Debt issued			$ 750,000,000
Debt issuance price (as a percent)			99.965%
Stated interest rate (as a percent)			3.45%
Deferred financing cost			$ 2,700,000
Long-term debt							748,225,000		747,655,000
Senior Notes \| 4.375% Senior Notes, due 2023
Debt Instrument [Line Items]
Debt issued					$ 250,000,000
Debt issuance price (as a percent)					99.879%
Stated interest rate (as a percent)					4.375%
Deferred financing cost					$ 2,600,000
Long-term debt							248,759,000		248,567,000
Line of Credit \| Revolving Credit Facility
Debt Instrument [Line Items]
Long-term debt							346,588,000		$ 853,124,000
Line of Credit \| 2018 Refinancing Agreement \| Revolving Credit Facility
Debt Instrument [Line Items]
Revolving credit facility, amount								$ 1,200,000,000.0
Uncommitted accordion feature, maximum								600,000,000.0
Line of credit facility, amount currently available							$ 1,170,300,000
Line of Credit \| 2018 Refinancing Agreement \| Term Loan
Debt Instrument [Line Items]
Revolving credit facility, amount								350,000,000.0
Line of Credit \| 2018 Refinancing Agreement \| Delayed Draw Term Loan
Debt Instrument [Line Items]
Revolving credit facility, amount								400,000,000.0
Line of Credit \| 2018 Refinancing Agreement \| Letter of Credit
Debt Instrument [Line Items]
Revolving credit facility, amount								450,000,000.0
Line of Credit \| 2018 Refinancing Agreement \| Swing Line Sub Facility
Debt Instrument [Line Items]
Revolving credit facility, amount								75,000,000.0
Line of Credit \| 2016 Refinancing Credit Agreement \| Revolving Credit Facility
Debt Instrument [Line Items]
Revolving credit facility, amount				$ 1,200,000,000.0
Credit facility loan term, years				5 years
Line of Credit \| 2016 Refinancing Credit Agreement \| Term Loan
Debt Instrument [Line Items]
Revolving credit facility, amount				$ 400,000,000.0
Bridge Loan \| 2018 Refinancing Agreement
Debt Instrument [Line Items]
Revolving credit facility, amount								$ 2,500,000,000.0
Scenario Forecast \| Senior Notes
Debt Instrument [Line Items]
Debt issuance price (as a percent)	101.00%