Change in benefit obligations and plan assets (Details) - Underfunded Plan [Member] - Qualified Plan [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	$ 436,307	$ 442,960
Service cost	0	0	$ 86
Interest cost	16,207	17,218	17,141
Plan participants' contributions	0	0
Actuarial (gain) loss	19,119	1,882
Benefits paid	(25,710)	(25,753)
Benefit obligation at end of year	445,923	436,307	442,960
Change in net plan assets:
Fair value of plan assets at beginning of year	333,509	332,667
Actual gain on plan assets	45,473	26,595
Employer contribution	1,112	0
Plan participants' contributions	0	0
Benefits paid	(25,710)	(25,753)
Fair value of net plan assets at end of year	354,384	333,509	332,667
Funded status - under	(91,539)	(102,798)
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	91,539	102,798
Benefit obligation liabilities - net amount recognized	(91,539)	(102,798)
Amounts recognized in accumulated other comprehensive (income) loss or regulatory assets (liabilities) consist of:
Actuarial loss	186,486	198,668
Prior service credit	0	0
Total	186,486	198,668
Other Postretirement Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	89,304	92,734
Service cost	1,508	1,647	1,816
Interest cost	3,265	3,688	3,607
Plan participants' contributions	1,368	1,405
Actuarial (gain) loss	1,781	(3,872)
Benefits paid	(6,020)	(6,298)
Benefit obligation at end of year	91,206	89,304	92,734
Change in net plan assets:
Fair value of plan assets at beginning of year	82,846	82,593
Actual gain on plan assets	9,612	4,184
Employer contribution	933	962
Plan participants' contributions	1,368	1,405
Benefits paid	(6,020)	(6,298)
Fair value of net plan assets at end of year	88,739	82,846	$ 82,593
Funded status - under	(2,467)	(6,458)
Amounts recognized in the Consolidated Balance Sheets at December 31:
Deferred charges and other assets - other	19,114	13,131
Other accrued liabilities	612	538
Deferred credits and other liabilities - other	20,969	19,051
Benefit obligation liabilities - net amount recognized	(2,467)	(6,458)
Amounts recognized in accumulated other comprehensive (income) loss or regulatory assets (liabilities) consist of:
Actuarial loss	13,423	17,470
Prior service credit	(11,632)	(13,003)
Total	$ 1,791	$ 4,467