Components of Net Periodic Benefit Cost (Tables)

6 Months Ended

Jun. 30, 2019

Retirement Benefits [Abstract]

Schedule of Net Periodic Benefit Costs

The service cost component of net periodic benefit costs included in the table below are components 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
	Period Ended June 30,
	Three Months						Six Months						Twelve Months
	2019			2018			2019			2018			2019			2018
(Thousands of dollars)
Service cost	$	6,466		$	7,139		$	12,932		$	14,278		$	27,209		$	25,974
Interest cost	12,251			11,044			24,503			22,087			46,590			45,129
Expected return on plan assets	(15,061		)	(14,688		)	(30,122		)	(29,377		)	(59,500		)	(56,975		)
Amortization of net actuarial loss	5,589			8,028			11,178			16,057			27,236			28,059
Net periodic benefit cost	$	9,245		$	11,523		$	18,491		$	23,045		$	41,535		$	42,187
	SERP
	Period Ended June 30,
	Three Months						Six Months						Twelve Months
	2019			2018			2019			2018			2019			2018
(Thousands of dollars)
Service cost	$	67		$	61		$	133		$	122		$	256		$	276
Interest cost	440			414			880			829			1,709			1,770
Amortization of net actuarial loss	255			376			510			751			1,261			1,472
Net periodic benefit cost	$	762		$	851		$	1,523		$	1,702		$	3,226		$	3,518
	PBOP
	Period Ended June 30,
	Three Months						Six Months						Twelve Months
	2019			2018			2019			2018			2019			2018
(Thousands of dollars)
Service cost	$	319		$	369		$	638		$	737		$	1,374		$	1,471
Interest cost	762			687			1,524			1,374			2,898			2,990
Expected return on plan assets	(789		)	(930		)	(1,578		)	(1,860		)	(3,436		)	(3,539		)
Amortization of prior service costs	318			334			635			668			1,302			1,335
Net periodic benefit cost	$	610		$	460		$	1,219		$	919		$	2,138		$	2,257