Commitments and Contingencies - Remaining Purchase Commitments and Estimated Capacity Fee Liability Requirements (Detail) € in Millions	Jun. 30, 2025 EUR (€)
Purchase Obligation Fiscal Year Maturity [Abstract]
Remaining Purchase Commitments, 2026	€ 4.1
Remaining Purchase Commitments, 2027	6.0
Remaining Purchase Commitments, 2028	6.0
Remaining Purchase Commitments, 2029	5.8
Remaining Purchase Commitments, 2030	5.8
Remaining Purchase Commitments, 2031	7.8
Remaining Purchase Commitments, 2032	7.7
Remaining Purchase Commitments, 2033	7.7
Remaining Purchase Commitments, 2034	4.3
Remaining Purchase Commitments, Total	55.2
Estimated Capacity Fees, 2025 (Remaining)	0.8
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	€ 8.8