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Employee Benefit Plans
 9 Months Ended
Sep. 30, 2025
Retirement Benefits [Abstract]  
Pension and Other Postretirement Benefits Disclosure
Defined Benefit Pension and Other Postretirement Benefit (OPEB) Plans

We sponsor several defined benefit pension and OPEB plans for our employees, including non-qualified pension plans. The U.S. qualified and non-qualified defined benefit pension plans comprise the majority of our total benefit obligation and benefit cost. We maintain a separate defined benefit plan for eligible employees in our U.K. operation. The U.S. defined benefit pension plans were frozen and closed to new entrants on December 31, 2013, the OPEB plan was frozen and closed to new entrants on December 31, 2012, and the U.K. plan was frozen and closed to new entrants on December 31, 2002.
The following table provides the components of the net periodic benefit cost for the defined benefit pension and OPEB plans.
Three Months Ended September 30
 Pension Benefits
 U.S. PlansU.K. PlanOPEB
 202520242025202420252024
(in millions of dollars)
Service Cost$2.3 $2.3 $— $— $— $— 
Interest Cost21.4 20.7 2.2 1.9 1.0 1.0 
Expected Return on Plan Assets(21.8)(22.8)(2.2)(2.1)(0.1)(0.1)
Amortization of:
Net Actuarial Loss (Gain)3.8 3.7 0.7 0.7 (0.3)(0.2)
   Prior Service Credit— — — — (0.1)(0.1)
Total Net Periodic Benefit Cost$5.7 $3.9 $0.7 $0.5 $0.5 $0.6 

Nine Months Ended September 30
 Pension Benefits  
 U.S. PlansU.K. PlanOPEB
 202520242025202420252024
(in millions of dollars)
Service Cost$7.0 $6.9 $— $— $— $— 
Interest Cost64.2 62.1 6.5 5.7 3.0 3.0 
Expected Return on Plan Assets(65.4)(68.4)(6.6)(6.3)(0.3)(0.3)
Amortization of:
Net Actuarial Loss (Gain)11.2 11.0 2.1 2.2 (0.9)(0.8)
   Prior Service Credit— — — — (0.2)(0.2)
Total Net Periodic Benefit Cost$17.0 $11.6 $2.0 $1.6 $1.6 $1.7 

We have made regulatory contributions of $0.3 million and $18.6 million to our U.K defined benefit pension plan during the three and nine months ended September 30, 2025, respectively.

The service cost component of net periodic pension and postretirement benefit cost is included as a component of compensation expense in our consolidated statements of income. All other components of net periodic pension and postretirement benefit cost are included in other expenses.