Commitments and Contingencies - Remaining Purchase Commitments and Estimated Capacity Fee Liability Requirements (Detail) - EUR (€) € in Millions	Dec. 31, 2023	Sep. 30, 2023
Purchase Obligation, Fiscal Year Maturity [Abstract]
Remaining Purchase Commitments, 2023		€ 2.4
Remaining Purchase Commitments, 2024	€ 2.9	14.6
Remaining Purchase Commitments, 2025		15.5
Remaining Purchase Commitments, 2026	4.2	19.4
Remaining Purchase Commitments, 2027	6.0	9.2
Remaining Purchase Commitments, 2028	6.0
Remaining Purchase Commitments, 2029	6.0
Remaining Purchase Commitments, 2030	6.0
Remaining Purchase Commitments, 2031	8.0
Remaining Purchase Commitments, 2032	8.0
Remaining Purchase Commitments, 2033	8.0
Remaining Purchase Commitments, 2034	4.4
Remaining Purchase Commitments, Total	59.5	€ 61.1
Estimated Capacity Fees, 2025	1.5
Estimated Capacity Fees, 2026	2.0
Estimated Capacity Fees, 2027	1.0
Estimated Capacity Fees, 2028	1.0
Estimated Capacity Fees, 2029	1.0
Estimated Capacity Fees, 2030	1.0
Estimated Capacity Fees, 2031	0.5
Estimated Capacity Fees, 2032	0.5
Estimated Capacity Fees, 2033	0.5
Estimated Capacity Fees, 2034	0.5
Estimated Capacity Fees, Total	€ 9.5