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Face (par) amount of debt instrument at time of issuance.

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Nominal or face amount used to calculate payments on the derivative liability.

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Number of Investors

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Long-Term Debt - Additional Information (Details)				1 Months Ended	3 Months Ended	6 Months Ended	12 Months Ended
Long-Term Debt - Additional Information (Details)	Nov. 03, 2016 USD ($)	Jun. 22, 2016 USD ($)	Dec. 19, 2013 USD ($)	Aug. 31, 2013 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Mar. 21, 2017 USD ($)	Jun. 05, 2014 USD ($)	Mar. 05, 2014 USD ($) investor	Jan. 12, 2012 USD ($)
Debt Instrument [Line Items]
Weighted average interest rate (as a percent)					2.43%	2.43%
Repayments of debt					$ 30,300,000		$ 39,600,000
Forward Starting Interest Rate Swap Agreement
Debt Instrument [Line Items]
Interest rate swap agreements, notional amount									$ 150,000,000		$ 150,000,000.0
Interest rate on notional value (as a percent)									2.56%		1.415%
Faiveley Transport
Debt Instrument [Line Items]
Acquired loans								$ 1,562,975,000
Senior Notes Due 2026
Debt Instrument [Line Items]
Stated interest rate (as a percent)					3.45%	3.45%	3.45%
Long-term debt					$ 747,580,000	$ 747,580,000	$ 747,474,000
4.375% Senior Notes, due 2023
Debt Instrument [Line Items]
Stated interest rate (as a percent)					4.375%	4.375%	4.375%
Long-term debt					$ 248,438,000	$ 248,438,000	$ 248,310,000
2016 Refinancing Credit Agreement
Debt Instrument [Line Items]
Deferred financing cost		$ 3,300,000
Minimum interest expense coverage ratio						3.0
Maximum debt to EBITDA ratio						3.25
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%
2016 Refinancing Credit Agreement \| Letter of Credit
Debt Instrument [Line Items]
Line of credit amount outstanding					$ 40,500,000	$ 40,500,000
2013 Refinancing Credit Agreement
Debt Instrument [Line Items]
Deferred financing cost			$ 1,000,000
Basis spread on effective rate					0.50%	0.50%
Debt instrument, basis spread on variable rate						1.00%
2013 Refinancing Credit Agreement \| Base Rate \| Minimum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate						0.00%
2013 Refinancing Credit Agreement \| Base Rate \| Maximum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate						0.75%
2013 Refinancing Credit Agreement \| Alternative Rate \| Minimum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate						0.75%
2013 Refinancing Credit Agreement \| Alternative Rate \| Maximum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate						1.75%
Schuldschein Loan
Debt Instrument [Line Items]
Long-term debt					$ 11,415,000	$ 11,415,000	98,671,000
Schuldschein Loan \| Faiveley Transport
Debt Instrument [Line Items]
Stated interest rate (as a percent)					4.00%	4.00%
Long-term debt					$ 11,400,000	$ 11,400,000
Acquired loans										$ 137,200,000
Number of investors \| investor										20
Repayments of debt						$ 125,800,000
Schuldschein Loan \| March 2021 \| Faiveley Transport
Debt Instrument [Line Items]
Original maturities						7 years
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	$ 2,600,000
Long-term debt					747,580,000	$ 747,580,000	747,474,000
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					248,438,000	248,438,000	248,310,000
Line of Credit \| Revolving Credit Facility
Debt Instrument [Line Items]
Long-term debt					966,235,000	966,235,000	$ 796,150,000
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 facility, amount currently available					$ 583,200,000	$ 583,200,000
Line of Credit \| 2016 Refinancing Credit Agreement \| Term Loan
Debt Instrument [Line Items]
Revolving credit facility, amount		$ 400,000,000.0
Line of Credit \| 2013 Refinancing Credit Agreement \| Revolving Credit Facility
Debt Instrument [Line Items]
Revolving credit facility, amount			$ 800,000,000
Credit facility loan term, years			5 years