Components of Net Periodic Benefit Cost (Tables)

9 Months Ended

Sep. 30, 2020

Retirement Benefits [Abstract]

Schedule of Net Periodic Benefit Costs

The service cost component of net periodic benefit costs included in the table below is a component of an overhead loading process associated with the cost of labor. The overhead process ultimately results in allocation of that portion of overall net periodic benefit costs to the same accounts to which productive labor is charged. As a result, service costs become components of various accounts, primarily operations and maintenance expense, net utility plant, and deferred charges and other assets for both the Company and Southwest. The other components of net periodic benefit cost are reflected in Other income (deductions) on the Condensed Consolidated Statements of Income of each entity.

	Qualified Retirement Plan
	September 30,
	Three Months				Nine Months				Twelve Months
	2020		2019		2020		2019		2020		2019
(Thousands of dollars)
Service cost	$	8,576	$	6,466	$	25,725	$	19,398	$	32,191	$	26,536
Interest cost	11,388		12,252		34,165		36,755		46,416		47,799
Expected return on plan assets	(16,324)		(15,061)		(48,972)		(45,183)		(64,033)		(59,872)
Amortization of net actuarial loss	9,006		5,589		27,019		16,767		32,608		24,796
Net periodic benefit cost	$	12,646	$	9,246	$	37,937	$	27,737	$	47,182	$	39,259
	SERP
	September 30,
	Three Months				Nine Months				Twelve Months
	2020		2019		2020		2019		2020		2019
(Thousands of dollars)
Service cost	$	97	$	66	$	292	$	199	$	359	$	261
Interest cost	401		440		1,204		1,320		1,644		1,734
Amortization of net actuarial loss	451		255		1,353		765		1,608		1,141
Net periodic benefit cost	$	949	$	761	$	2,849	$	2,284	$	3,611	$	3,136
	PBOP
	September 30,
	Three Months				Nine Months				Twelve Months
	2020		2019		2020		2019		2020		2019
(Thousands of dollars)
Service cost	$	395	$	319	$	1,186	$	957	$	1,505	$	1,325
Interest cost	646		761		1,936		2,285		2,697		2,972
Expected return on plan assets	(852)		(789)		(2,556)		(2,367)		(3,345)		(3,296)
Amortization of prior service costs	289		318		867		953		1,185		1,286
Net periodic benefit cost	$	478	$	609	$	1,433	$	1,828	$	2,042	$	2,287