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Long-Term Debt - Additional Information (Details)			1 Months Ended	3 Months Ended	5 Months Ended	6 Months Ended
Long-Term Debt - Additional Information (Details)	Nov. 03, 2016 USD ($)	Jun. 22, 2016 USD ($)	Aug. 31, 2013 USD ($)	Mar. 31, 2018	Nov. 25, 2016	Jun. 30, 2018 USD ($)	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]
Weighted average interest rate (as a percent)						3.03%
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%
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						$ 33,600,000
Senior Notes Due 2026
Debt Instrument [Line Items]
Stated interest rate (as a percent)						3.45%
Long-term debt						$ 747,788,000		$ 747,655,000
4.375% Senior Notes, due 2023
Debt Instrument [Line Items]
Stated interest rate (as a percent)						4.375%
Long-term debt						$ 248,695,000		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						$ 11,681,000		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						11,700,000
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						747,788,000		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						248,695,000		248,567,000
Line of Credit \| Revolving Credit Facility
Debt Instrument [Line Items]
Long-term debt						865,165,000		$ 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						$ 647,900,000
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