Fair value - other postretirement (Details 10) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Percentage investment in common stock of small-cap companies	10.00%	10.00%
Other Postretirement Benefits | Qualified Plan | Overfunded Plan
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	$ 94,587	$ 82,516	$ 88,739
Percentage investment in corporate bonds	50.00%	51.00%
Percentage investment in common stock of large-cap U.S. companies	25.00%	23.00%
Percentage Investment in US Government Securities	7.00%	7.00%
Percentage investment in common stock of small-cap companies	7.00%	7.00%
Percentage investment in other investments	11.00%	12.00%
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	$ 4,017	$ 3,866
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Equity securities of U.S. companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	2,073	1,767
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Equities securities of International companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	1	2
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Insurance investment contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	88,496	76,881
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Total assets measured at fair value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	94,587	82,516
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities of U.S. companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	2,073	1,767
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Insurance investment contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	10	1
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Total assets measured at fair value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	2,083	1,768
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Significant Other Observable Inputs (Level 2) | Cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	4,017	3,866
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Significant Other Observable Inputs (Level 2) | Equities securities of International companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	1	2
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Significant Other Observable Inputs (Level 2) | Insurance investment contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	88,486	76,880
Other Postretirement Benefits | Qualified Plan | Overfunded Plan | Significant Other Observable Inputs (Level 2) | Total assets measured at fair value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	$ 92,504	$ 80,748