Fair value - pension (Details 9) - Qualified Plan - Underfunded Plan - Pension Benefits - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	$ 365,264	$ 307,809	$ 354,384
Percentage investment in common stock of international companies	29.00%	27.00%
Percentage investment in common stock of large-cap U.S. companies	21.00%	18.00%
Percentage Investment in US Government Securities	18.00%
Percentage investment in corporate bonds	9.00%	31.00%
Percentage investment in cash equivalents	6.00%	5.00%
Percentage investment in other investments	17.00%	19.00%
Cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	$ 26,166	$ 4,930
Equity securities of U.S. companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	14,457	11,038
Equities securities of International companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	938	967
Collective and mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	219,800	197,560
Corporate debt securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	80,768	73,110
Municipal bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	11,828	10,624
U.S. Government securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	9,378	6,375
Total assets measured at fair value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	363,335	304,604
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	14,457	11,038
Quoted Prices in Active Markets for Identical Assets (Level 1) | Collective and mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	160,906	145,960
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Government securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	7,296	479
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	182,659	157,477
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	26,166	4,930
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	938	967
Significant Other Observable Inputs (Level 2) | Collective and mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	58,894	51,600
Significant Other Observable Inputs (Level 2) | Corporate debt securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	80,768	73,110
Significant Other Observable Inputs (Level 2) | Municipal bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	11,828	10,624
Significant Other Observable Inputs (Level 2) | U.S. Government securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	2,082	5,896
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	$ 180,676	$ 147,127