Forecasted revenue from contract agreement.

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Number of long term debt facilities.

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Amount of amortization expense attributable to debt issuance costs.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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No definition available.

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

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Fair value as of the balance sheet date of derivative instrument not designated as hedging instrument and classified as a liability.

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- Definition

Nominal or face amount used to calculate payment on derivative.

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

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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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- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

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- Definition

Period in which remaining performance obligation is expected to be recognized as revenue, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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- Definition

The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

+ References

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The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

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- Definition

The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE).

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Variable Interest Entities - Narrative (Details) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020 USD ($)	Mar. 31, 2021 USD ($) item	Mar. 31, 2020 USD ($)	Mar. 31, 2021 USD ($) item	Mar. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Mar. 31, 2018 USD ($)
Revenue recognition
Termination costs			$ (16,090)		$ (16,090)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Revenue recognition
Contract term		12 months		12 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Revenue recognition
Contract term		24 months		24 months
Revolving Credit Agreement
Revenue recognition
Maximum borrowing capacity under credit agreement							$ 800,000
Design and Build Phase \| Boston OpCo Amended Credit Agreement \| Long-term Debt Facility \| LIBOR \| Minimum
Revenue recognition
Variable interest rate (as a percent)				1.75%
Design and Build Phase \| Boston OpCo Amended Credit Agreement \| Long-term Debt Facility \| LIBOR \| Maximum
Revenue recognition
Variable interest rate (as a percent)				2.00%
MBTA \| Design and Build Phase \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01
Revenue recognition
Contract term		4 years		4 years
MBTA \| Operate and Maintain Phase \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-10-01
Revenue recognition
Contract term		12 years		12 years
P3 Joint Venture
Revenue recognition
Maximum exposure to loss		$ 2,900		$ 2,900
OpCo. \| Interest Rate Swaps \| Not Designated as Hedging Instrument
Revenue recognition
Notional principal outstanding derivative instruments		233,500		233,500		$ 194,000
Unrealized gain (loss) on derivative		15,100	$ 15,800	18,300	$ 11,500
OpCo. \| Interest Rate Swaps \| Not Designated as Hedging Instrument \| Other noncurrent liabilities
Revenue recognition
Fair value interest rate swaps		12,500		12,500		5,900
OpCo. \| P3 Credit Agreement \| Revolving Credit Agreement
Revenue recognition
Maximum borrowing capacity under credit agreement		15,800		15,800
OpCo. \| P3 Credit Agreement \| Long-term Debt Facility
Revenue recognition
Maximum borrowing capacity under credit agreement		$ 421,600		$ 421,600
OpCo. \| Boston OpCo Amended Credit Agreement
Revenue recognition
Number of long-term debt facilities \| item		2		2
Debt issuance and modification costs						9,200
OpCo. \| Boston OpCo Amended Credit Agreement \| Interest Rate Swaps
Revenue recognition
Termination costs	$ 34,400
OpCo. \| Boston OpCo Amended Credit Agreement \| Long-term Debt Facility
Revenue recognition
Unamortized deferred financing costs		$ 16,700		$ 16,700		$ 17,200
OpCo. \| Original MBTA Contract
Revenue recognition
Expected contract revenue		175,800		175,800
OpCo. \| Original MBTA Contract \| Design and Build Phase
Revenue recognition
Expected contract revenue		43,500		43,500
OpCo. \| Original MBTA Contract \| Operate and Maintain Phase
Revenue recognition
Expected contract revenue		618,000		618,000
OpCo. \| MBTA
Revenue recognition
Expected contract revenue		$ 596,400		$ 596,400
Cubic Transportation Systems \| HoldCo.
Revenue recognition
Ownership percentage		10.00%		10.00%
Cubic Transportation Systems \| HoldCo. \| Letter of credit agreement
Revenue recognition
Maximum borrowing capacity under credit agreement								$ 2,900
Borrowings outstanding under credit agreement		$ 0		$ 0
John Laing \| HoldCo.
Revenue recognition
Ownership percentage		90.00%		90.00%
John Laing \| HoldCo. \| Equity Bridge Loan
Revenue recognition
Loan amount								$ 26,200