Fair Value Measurements - Recurring Fair Value Measurements (Details) - Recurring - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
ASSETS:
Cash equivalents	$ 15.8	$ 5.6
Trust assets	0.6	0.1
Derivative instruments	12.4	129.1
TOTAL	28.8	134.8
LIABILITIES:
Derivative instruments	(10.4)	31.0
TOTAL	(10.4)	31.0
Level 1
ASSETS:
Cash equivalents	15.8	5.6
Trust assets	0.6	0.1
Derivative instruments	0.0	0.0
TOTAL	16.4	5.7
LIABILITIES:
Derivative instruments	0.0	0.0
TOTAL	0.0	0.0
Level 2
ASSETS:
Cash equivalents	0.0	0.0
Trust assets	0.0	0.0
Derivative instruments	12.4	127.3
TOTAL	12.4	127.3
LIABILITIES:
Derivative instruments	(10.4)	26.4
TOTAL	(10.4)	26.4
Level 3
ASSETS:
Cash equivalents	0.0	0.0
Trust assets	0.0	0.0
Derivative instruments	0.0	1.8
TOTAL	0.0	1.8
LIABILITIES:
Derivative instruments	0.0	4.6
TOTAL	$ 0.0	$ 4.6