Fair value - other postretirement (Details 10) - Other Postretirement Benefits - Qualified Plan - Overfunded Plan - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	$ 101,639	$ 94,587	$ 82,516
Defined Benefit Plan, Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Postretirement Investments in Collective and Mutual Funds	8.00%	6.00%
Postretirement Investments in Insurance Contracts	1.00%
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	$ 3,517	$ 4,017
Defined Benefit Plan, Equity Securities, US [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	$ 1,850	$ 2,073
Defined Benefit Plan, Equity Securities, Non-US [Member]
Defined Benefit Plan Disclosure [Line Items]
Postretirement Investments in Collective and Mutual Funds	24.00%	29.00%
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	$ 2	$ 1
Collective and mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	157	231
Insurance investment contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	96,103	88,265
Total assets measured at fair value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	$ 101,639	$ 94,587
Corporate debt securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Postretirement Investments in Collective and Mutual Funds	36.00%	9.00%
Postretirement Investments in Insurance Contracts	67.00%	50.00%
Defined Benefit Plan, Equity Securities, US, Large Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Postretirement Investments in Collective and Mutual Funds	18.00%	21.00%
Postretirement Investments in Insurance Contracts	10.00%	25.00%
Defined Benefit Plan, Equity Securities, US, Small Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Postretirement Investments in Insurance Contracts	4.00%	7.00%
Other Investments
Defined Benefit Plan Disclosure [Line Items]
Postretirement Investments in Collective and Mutual Funds	9.00%	17.00%
Postretirement Investments in Insurance Contracts	6.00%	11.00%
U.S. Government securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Postretirement Investments in Collective and Mutual Funds	5.00%	18.00%
Postretirement Investments in Insurance Contracts	12.00%	7.00%
Quoted Prices in Active Markets for Identical Assets  (Level 1) | Defined Benefit Plan, Equity Securities, US [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	$ 1,850	$ 2,073
Quoted Prices in Active Markets for Identical Assets  (Level 1) | Collective and mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	10	10
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	1,860	2,083
Significant Other Observable Inputs  (Level 2) | Defined Benefit Plan, Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	3,517	4,017
Significant Other Observable Inputs  (Level 2) | Defined Benefit Plan, Equity Securities, Non-US [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	2	1
Significant Other Observable Inputs  (Level 2) | Collective and mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	147	221
Significant Other Observable Inputs  (Level 2) | Insurance investment contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	96,103	88,265
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	99,769	$ 92,504
Fair Value Measured at Net Asset Value Per Share | Collective and mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	$ 10