Change in benefit obligations and plan assets (Details) - Qualified Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Underfunded Plan | Pension Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	$ 411,497	$ 437,360
Service cost	0	0	$ 0
Interest cost	10,522	9,819	12,093
Plan participants' contributions	0	0
Actuarial gain	(85,303)	(12,140)
Benefits paid	(24,672)	(23,542)
Benefit obligation at end of year	312,044	411,497	437,360
Change in net plan assets:
Fair value of plan assets at beginning of year	373,109	383,834
Actual return on plan assets	(77,975)	12,817
Employer contribution	0	0
Plan participants' contributions	0	0
Benefits paid	(24,672)	(23,542)
Fair value of net plan assets at end of year	270,462	373,109	383,834
Funded status - (under) over	(41,582)	(38,388)
Amounts recognized in the Consolidated Balance Sheets at December 31:
Noncurrent assets - other	0	0
Other accrued liabilities	0	0
Noncurrent liabilities - other	41,582	38,388
Benefit obligation (liabilities) assets - net amount recognized	(41,582)	(38,388)
Amounts recognized in accumulated other comprehensive loss:
Actuarial loss (gain)	32,378	25,976
Prior service credit	0	0
Total	32,378	25,976
Amounts recognized in regulatory assets or liabilities:
Actuarial loss (gain)	141,207	142,166
Prior service credit	0	0
Total	141,207	142,166
Overfunded Plan | Other Postretirement Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	73,460	86,155
Service cost	1,416	1,600	1,532
Interest cost	1,896	1,862	2,437
Plan participants' contributions	569	641
Actuarial gain	(18,401)	(12,802)
Benefits paid	(4,009)	(3,996)
Benefit obligation at end of year	54,931	73,460	86,155
Change in net plan assets:
Fair value of plan assets at beginning of year	100,158	101,639
Actual return on plan assets	(20,893)	1,398
Employer contribution	501	476
Plan participants' contributions	569	641
Benefits paid	(4,009)	(3,996)
Fair value of net plan assets at end of year	76,326	100,158	$ 101,639
Funded status - (under) over	21,395	26,698
Amounts recognized in the Consolidated Balance Sheets at December 31:
Noncurrent assets - other	36,325	45,863
Other accrued liabilities	1,044	544
Noncurrent liabilities - other	13,886	18,621
Benefit obligation (liabilities) assets - net amount recognized	21,395	26,698
Amounts recognized in accumulated other comprehensive loss:
Actuarial loss (gain)	(2,923)	2,367
Prior service credit	(289)	(290)
Total	(3,212)	2,077
Amounts recognized in regulatory assets or liabilities:
Actuarial loss (gain)	(1,439)	(14,727)
Prior service credit	(3,796)	(5,193)
Total	$ (5,235)	$ (19,920)