Change in benefit obligations and plan assets (Details) - Qualified Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Underfunded Plan | Pension Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	$ 391,602	$ 445,923
Service cost	0	0	$ 0
Interest cost	15,225	14,591	16,207
Plan participants' contributions	0	0
Actuarial (gain) loss	40,219	(32,637)
Benefits paid	(25,880)	(36,275)
Benefit obligation at end of year	421,166	391,602	445,923
Change in net plan assets:
Fair value of plan assets at beginning of year	307,809	354,384
Actual gain (loss) on plan assets	58,409	(21,138)
Employer contribution	24,926	10,838
Plan participants' contributions	0	0
Benefits paid	(25,880)	(36,275)
Fair value of net plan assets at end of year	365,264	307,809	354,384
Funded status - over (under)	(55,902)	(83,793)
Amounts recognized in the Consolidated Balance Sheets at December 31:
Deferred charges and other assets - other	0	0
Other accrued liabilities	0	0
Deferred credits and other liabilities - other	55,902	83,793
Benefit obligation assets (liabilities) - net amount recognized	(55,902)	(83,793)
Amounts recognized in accumulated other comprehensive loss:
Actuarial loss	27,748	28,796
Prior service credit	0	0
Total	27,748	28,796
Amounts recognized in regulatory assets or liabilities:
Actuarial (gain) loss	155,484	159,939
Prior service credit	0	0
Total	155,484	159,939
Overfunded Plan | Other Postretirement Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	81,201	91,206
Service cost	1,142	1,494	1,508
Interest cost	2,986	2,899	3,265
Plan participants' contributions	1,040	1,282
Actuarial (gain) loss	2,632	(10,115)
Benefits paid	(5,387)	(5,565)
Benefit obligation at end of year	83,614	81,201	91,206
Change in net plan assets:
Fair value of plan assets at beginning of year	82,516	88,739
Actual gain (loss) on plan assets	15,731	(2,781)
Employer contribution	687	842
Plan participants' contributions	1,040	1,281
Benefits paid	(5,387)	(5,565)
Fair value of net plan assets at end of year	94,587	82,516	$ 88,739
Funded status - over (under)	10,973	1,315
Amounts recognized in the Consolidated Balance Sheets at December 31:
Deferred charges and other assets - other	30,475	20,843
Other accrued liabilities	647	660
Deferred credits and other liabilities - other	18,855	18,868
Benefit obligation assets (liabilities) - net amount recognized	10,973	1,315
Amounts recognized in accumulated other comprehensive loss:
Actuarial loss	6,118	6,372
Prior service credit	(731)	(848)
Total	5,387	5,524
Amounts recognized in regulatory assets or liabilities:
Actuarial (gain) loss	(4,450)	3,944
Prior service credit	(8,109)	(9,390)
Total	$ (12,559)	$ (5,446)