Fair value - other postretirement (Details 9) - Other Postretirement Benefits - Qualified Plan [Member] - Underfunded Plan [Member] - USD ($)	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	$ 88,739,000		$ 82,846,000		$ 82,593,000
Percentage investment in corporate bonds	38.00%		38.00%
Percentage investment in common stock of large-cap U.S. companies	23.00%		25.00%
Percentage investment in US Government securities	21.00%		20.00%
Percentage investment in mortgage backed securities	9.00%		9.00%
Percentage investment in other investments	9.00%		8.00%
Estimated future employer contributions in next fiscal year	$ 800,000
Cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	4,815,000		$ 250,000
Equity securities of U.S. companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	2,316,000		2,328,000
Equities securities of International companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	4,000		5,000
Insurance investment contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	81,604,000	[1]	80,263,000	[2]
Total assets measured at fair value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	88,739,000		82,846,000
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
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	2,316,000		2,328,000
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equities securities of International companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	4,000		5,000
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	3,000	[1]	0	[2]
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	2,323,000		2,333,000
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	4,815,000		250,000
Significant Other Observable Inputs (Level 2) | Equity securities of U.S. companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
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	0		0
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	81,601,000	[1]	80,263,000	[2]
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	86,416,000		80,513,000
Significant Unobservable Inputs (Level 3) | Cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Significant Unobservable Inputs (Level 3) | Equity securities of U.S. companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Significant Unobservable Inputs (Level 3) | Equities securities of International companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Significant Unobservable Inputs (Level 3) | Insurance investment contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0	[1]	0	[2]
Significant Unobservable Inputs (Level 3) | Total assets measured at fair value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	$ 0		$ 0

[1] The insurance contract invests approximately 38 percent in corporate bonds, 23 percent in common stock of large-cap U.S. companies, 21 percent in U.S. Government securities, 9 percent in mortgage-backed securities and 9 percent in other investments.

[2] The insurance contract invests approximately 38 percent in corporate bonds, 25 percent in common stock of large-cap U.S. companies, 20 percent in U.S. Government securities, 9 percent in mortgage-backed securities and 8 percent in other investments.