Stockholders' Equity - Changes in Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 3,997.3
Other comprehensive income	27.5	$ 31.5	$ (27.2)
Ending balance	4,404.0	3,997.3
Unfunded Employee Benefit Obligations
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(70.7)	(100.3)
Other comprehensive income before reclassifications	23.1	23.2
Amounts reclassified from AOCI	4.1	6.4
Other comprehensive income	27.2	29.6
Ending balance	(43.5)	(70.7)	(100.3)
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(70.9)	(102.4)
Other comprehensive income before reclassifications	23.4	25.0
Amounts reclassified from AOCI	4.1	6.5
Other comprehensive income	27.5	31.5
Ending balance	(43.4)	(70.9)	(102.4)
Unrealized Loss on Foreign Exchange Contracts | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(0.1)	(0.2)
Other comprehensive income before reclassifications	0.0	0.0
Amounts reclassified from AOCI	0.0	0.1
Other comprehensive income	0.0	0.1
Ending balance	(0.1)	(0.1)	(0.2)
Unrealized Loss (Gain) on Marketable Debt Securities | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(0.1)	(1.9)
Other comprehensive income before reclassifications	0.3	1.8
Amounts reclassified from AOCI	0.0	0.0
Other comprehensive income	0.3	1.8
Ending balance	$ 0.2	$ (0.1)	$ (1.9)