Fair value - pension (Details 9) - Qualified Plan - Underfunded Plan - Pension Benefits - 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	$ 383,834	$ 365,264	$ 307,809
Defined Benefit Plan, Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Percentage	8.00%	6.00%
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	$ 7,841	$ 26,166
Defined Benefit Plan, Equity Securities, US [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	$ 12,844	$ 14,457
Defined Benefit Plan, Equity Securities, Non-US [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Percentage	24.00%	29.00%
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	$ 1,727	$ 938
Collective and mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	$ 233,185	$ 219,800
Corporate debt securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Percentage	36.00%	9.00%
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	$ 92,809	$ 80,768
Municipal bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	$ 10,126	$ 11,828
U.S. Government securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Percentage	5.00%	18.00%
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	$ 13,872	$ 9,378
Total assets measured at fair value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	$ 383,834	$ 363,335
Defined Benefit Plan, Equity Securities, US, Large Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Percentage	18.00%	21.00%
Other Investments
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Percentage	9.00%	17.00%
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	$ 12,844	$ 14,457
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	177,397	160,906
Quoted Prices in Active Markets for Identical Assets  (Level 1) | U.S. Government securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	11,177	7,296
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	201,418	182,659
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	7,841	26,166
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	1,727	938
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	55,788	58,894
Significant Other Observable Inputs  (Level 2) | Corporate debt securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	92,809	80,768
Significant Other Observable Inputs  (Level 2) | Municipal bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	10,126	11,828
Significant Other Observable Inputs  (Level 2) | U.S. Government securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount	2,695	2,082
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	170,986	$ 180,676
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	$ 11,430