Change in benefit obligations and plan assets (Details) - Underfunded Plan [Member] - Qualified Plan [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	$ 445,923	$ 436,307
Service cost	0	0	$ 0
Interest cost	14,591	16,207	17,218
Plan participants' contributions	0	0
Actuarial (gain) loss	(32,637)	19,119
Benefits paid	(36,275)	(25,710)
Benefit obligation at end of year	391,602	445,923	436,307
Change in net plan assets:
Fair value of plan assets at beginning of year	354,384	333,509
Actual gain (loss) on plan assets	(21,138)	45,473
Employer contribution	10,838	1,112
Plan participants' contributions	0	0
Benefits paid	(36,275)	(25,710)
Fair value of net plan assets at end of year	307,809	354,384	333,509
Funded status - over (under)	(83,793)	(91,539)
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	83,793	91,539
Benefit obligation assets (liabilities) - net amount recognized	(83,793)	(91,539)
Amounts recognized in accumulated other comprehensive (income) loss or regulatory assets (liabilities) consist of:
Actuarial loss	188,735	186,486
Prior service credit	0	0
Total	188,735	186,486
Other Postretirement Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	91,206	89,304
Service cost	1,494	1,508	1,647
Interest cost	2,899	3,265	3,688
Plan participants' contributions	1,282	1,368
Actuarial (gain) loss	(10,115)	1,781
Benefits paid	(5,565)	(6,020)
Benefit obligation at end of year	81,201	91,206	89,304
Change in net plan assets:
Fair value of plan assets at beginning of year	88,739	82,846
Actual gain (loss) on plan assets	(2,781)	9,612
Employer contribution	842	933
Plan participants' contributions	1,281	1,368
Benefits paid	(5,565)	(6,020)
Fair value of net plan assets at end of year	82,516	88,739	$ 82,846
Funded status - over (under)	1,315	(2,467)
Amounts recognized in the Consolidated Balance Sheets at December 31:
Deferred charges and other assets - other	20,843	19,114
Other accrued liabilities	660	612
Deferred credits and other liabilities - other	18,868	20,969
Benefit obligation assets (liabilities) - net amount recognized	1,315	(2,467)
Amounts recognized in accumulated other comprehensive (income) loss or regulatory assets (liabilities) consist of:
Actuarial loss	10,316	13,423
Prior service credit	(10,238)	(11,632)
Total	$ 78	$ 1,791