Change in benefit obligations and plan assets (Details) - Qualified Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Underfunded Plan | Pension Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	$ 421,166	$ 391,602
Service cost	0	0	$ 0
Interest cost	12,093	15,225	14,591
Plan participants' contributions	0	0
Actuarial loss	27,737	40,219
Benefits paid	(23,636)	(25,880)
Benefit obligation at end of year	437,360	421,166	391,602
Change in net plan assets:
Fair value of plan assets at beginning of year	365,264	307,809
Actual return on plan assets	42,206	58,409
Employer contribution	0	24,926
Plan participants' contributions	0	0
Benefits paid	(23,636)	(25,880)
Fair value of net plan assets at end of year	383,834	365,264	307,809
Funded status - over (under)	(53,526)	(55,902)
Amounts recognized in the Consolidated Balance Sheets at December 31:
Noncurrent assets - other	0	0
Other accrued liabilities	0	0
Noncurrent liabilities - other	53,526	55,902
Benefit obligation assets (liabilities) - net amount recognized	(53,526)	(55,902)
Amounts recognized in accumulated other comprehensive loss:
Actuarial loss	27,527	27,748
Prior service credit	0	0
Total	27,527	27,748
Amounts recognized in regulatory assets or liabilities:
Actuarial (gain) loss	154,013	155,484
Prior service credit	0	0
Total	154,013	155,484
Overfunded Plan | Other Postretirement Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	83,614	81,201
Service cost	1,532	1,142	1,494
Interest cost	2,437	2,986	2,899
Plan participants' contributions	752	1,040
Actuarial loss	2,203	2,632
Benefits paid	(4,383)	(5,387)
Benefit obligation at end of year	86,155	83,614	81,201
Change in net plan assets:
Fair value of plan assets at beginning of year	94,587	82,516
Actual return on plan assets	10,249	15,731
Employer contribution	434	687
Plan participants' contributions	752	1,040
Benefits paid	(4,383)	(5,387)
Fair value of net plan assets at end of year	101,639	94,587	$ 82,516
Funded status - over (under)	15,484	10,973
Amounts recognized in the Consolidated Balance Sheets at December 31:
Noncurrent assets - other	36,769	30,475
Other accrued liabilities	622	647
Noncurrent liabilities - other	20,663	18,855
Benefit obligation assets (liabilities) - net amount recognized	15,484	10,973
Amounts recognized in accumulated other comprehensive loss:
Actuarial loss	5,557	6,118
Prior service credit	(634)	(731)
Total	4,923	5,387
Amounts recognized in regulatory assets or liabilities:
Actuarial (gain) loss	(8,228)	(4,450)
Prior service credit	(6,808)	(8,109)
Total	$ (15,036)	$ (12,559)