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Benefit Plans and Other Postretirement Benefits (Tables)
 9 Months Ended
Sep. 30, 2018
Retirement Benefits [Abstract]  
Components of Net Periodic Benefit Cost (Credit)
Components of Net Periodic Benefit Cost (Credit)
 
 
Three Months Ended Sept. 30
 
 
2018
 
2017
 
2018
 
2017
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health
Care Benefits
Service cost
 
$
2,429

 
$
2,439

 
$
280

 
$
219

Interest cost (a)
 
4,603

 
4,928

 
410

 
415

Expected return on plan assets (a)
 
(7,082
)
 
(6,971
)
 
(615
)
 
(589
)
Amortization of prior service credit (a)
 
(34
)
 
—

 
(101
)
 
(100
)
Amortization of net loss (gain) (a)
 
3,517

 
3,245

 
(114
)
 
(155
)
Net periodic benefit cost (credit)
 
3,433

 
3,641

 
(140
)
 
(210
)
(Costs) credits not recognized due to the effects of regulation
 
(468
)
 
553

 
—

 
—

Net benefit cost (credit) recognized for financial reporting
 
$
2,965

 
$
4,194

 
$
(140
)
 
$
(210
)


 
 
Nine Months Ended Sept. 30
 
 
2018
 
2017
 
2018
 
2017
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health
Care Benefits
Service cost
 
$
7,289

 
$
7,319

 
$
839

 
$
657

Interest cost (a)
 
13,808

 
14,783

 
1,231

 
1,245

Expected return on plan assets (a)
 
(21,246
)
 
(20,913
)
 
(1,846
)
 
(1,767
)
Amortization of prior service credit (a)
 
(103
)
 
—

 
(302
)
 
(300
)
Amortization of net loss (gain) (a)
 
10,551

 
9,735

 
(340
)
 
(465
)
Net periodic benefit cost (credit)
 
10,299

 
10,924

 
(418
)
 
(630
)
Credits not recognized due to the effects of regulation
 
1,267

 
1,275

 
—

 
—

Net benefit cost (credit) recognized for financial reporting
 
$
11,566

 
$
12,199

 
$
(418
)
 
$
(630
)

(a) The components of net periodic cost other than the service cost component are included in the line item “other expense, net” in the income statement or capitalized on the balance sheet as a regulatory asset.