REVENUE FROM CONTRACTS WITH CUSTOMERS - Performance Obligation (Details) $ in Millions	Mar. 31, 2025 USD ($)
Disaggregation of Revenue [Line Items]
Fixed performance obligations	$ 278.0
APCo
Disaggregation of Revenue [Line Items]
Fixed performance obligations	79.4
I&M
Disaggregation of Revenue [Line Items]
Fixed performance obligations	21.3
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-04-01
Disaggregation of Revenue [Line Items]
Fixed performance obligations	$ 70.2
Fixed performance obligations, period	9 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-04-01 | APCo
Disaggregation of Revenue [Line Items]
Fixed performance obligations	$ 12.0
Fixed performance obligations, period	9 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-04-01 | I&M
Disaggregation of Revenue [Line Items]
Fixed performance obligations	$ 3.3
Fixed performance obligations, period	9 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-01-01
Disaggregation of Revenue [Line Items]
Fixed performance obligations	$ 143.2
Fixed performance obligations, period	2 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-01-01 | APCo
Disaggregation of Revenue [Line Items]
Fixed performance obligations	$ 31.9
Fixed performance obligations, period	2 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-01-01 | I&M
Disaggregation of Revenue [Line Items]
Fixed performance obligations	$ 8.8
Fixed performance obligations, period	2 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2028-01-01
Disaggregation of Revenue [Line Items]
Fixed performance obligations	$ 48.6
Fixed performance obligations, period	2 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2028-01-01 | APCo
Disaggregation of Revenue [Line Items]
Fixed performance obligations	$ 24.0
Fixed performance obligations, period	2 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2028-01-01 | I&M
Disaggregation of Revenue [Line Items]
Fixed performance obligations	$ 6.8
Fixed performance obligations, period	2 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2030-01-01
Disaggregation of Revenue [Line Items]
Fixed performance obligations	$ 16.0
Fixed performance obligations, period
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2030-01-01 | APCo
Disaggregation of Revenue [Line Items]
Fixed performance obligations	$ 11.5
Fixed performance obligations, period
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2030-01-01 | I&M
Disaggregation of Revenue [Line Items]
Fixed performance obligations	$ 2.4
Fixed performance obligations, period