Fair value - pension and postretirement (Details 8) - USD ($)	12 Months Ended
	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	$ 333,509,000		$ 332,667,000		$ 354,363,000
Percentage investment in common stock of large-cap U.S. companies	20.00%		19.00%
Percentage investment in US Government securities	7.00%
Percentage investment in corporate bonds	21.00%		16.00%
Percentage investment in common stock of international companies	29.00%		29.00%
Percentage investment in cash equivalents	8.00%		16.00%
Percentage investment in common stock of mid-cap U.S. companies			6.00%
Percentage investment in other investments	15.00%		14.00%
Estimated future employer contributions in next fiscal year	$ 2,000,000
Pension Benefits | Cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	6,347,000		$ 8,379,000
Pension Benefits | Equity securities of U.S. companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	11,348,000		15,135,000
Pension Benefits | Equities Securities of International companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	1,584,000		2,332,000
Pension Benefits | Collective and Mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	226,107,000	[1]	217,968,000	[2]
Pension Benefits | Corporate debt securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	68,677,000		62,145,000
Pension Benefits | Municipal bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	11,002,000		11,680,000
Pension Benefits | U.S. Government securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	6,396,000		12,111,000
Pension Benefits | Total assets measured at fair value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	331,461,000		329,750,000
Pension Benefits | 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
Pension Benefits | 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	11,348,000		15,135,000
Pension Benefits | 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	1,584,000		2,332,000
Pension Benefits | 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	162,055,000	[1]	154,400,000	[2]
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate debt securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipal bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Pension Benefits | 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	4,352,000		5,288,000
Pension Benefits | 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	179,339,000		177,155,000
Pension Benefits | 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	6,347,000		8,379,000
Pension Benefits | 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
Pension Benefits | 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
Pension Benefits | 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	64,052,000	[1]	63,568,000	[2]
Pension Benefits | 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	68,677,000		62,145,000
Pension Benefits | 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	11,002,000		11,680,000
Pension Benefits | 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	2,044,000		6,823,000
Pension Benefits | 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	152,122,000		152,595,000
Pension Benefits | 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
Pension Benefits | 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
Pension Benefits | 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
Pension Benefits | Significant Unobservable Inputs (Level 3) | Collective and Mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0	[1]	0	[2]
Pension Benefits | Significant Unobservable Inputs (Level 3) | Corporate debt securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Pension Benefits | Significant Unobservable Inputs (Level 3) | Municipal bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Pension Benefits | Significant Unobservable Inputs (Level 3) | U.S. Government securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Pension Benefits | 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
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	$ 82,846,000		$ 82,593,000		$ 87,586,000
Percentage investment in common stock of large-cap U.S. companies	25.00%		19.00%
Percentage investment in US Government securities	20.00%		22.00%
Percentage investment in corporate bonds	38.00%		36.00%
Percentage investment in other investments	8.00%		13.00%
Percentage investment in mortgage backed securities	9.00%		10.00%
Estimated future employer contributions in next fiscal year	$ 900,000
Other Postretirement Benefits | Cash equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	250,000		$ 3,261,000
Other Postretirement Benefits | Equity securities of U.S. companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	2,328,000		2,274,000
Other Postretirement Benefits | Equities Securities of International companies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	5,000		9,000
Other Postretirement Benefits | Insurance investment contract
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	80,263,000	[3]	77,044,000	[4]
Other Postretirement Benefits | Total assets measured at fair value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	82,846,000		82,588,000
Other Postretirement Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	2,333,000		2,283,000
Other Postretirement Benefits | 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
Other Postretirement Benefits | 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	2,328,000		2,274,000
Other Postretirement Benefits | 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	5,000		9,000
Other Postretirement Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Insurance investment contract
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0	[3]	0	[4]
Other Postretirement Benefits | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	80,513,000		80,305,000
Other Postretirement Benefits | 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	250,000		3,261,000
Other Postretirement Benefits | 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
Other Postretirement Benefits | 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
Other Postretirement Benefits | Significant Other Observable Inputs (Level 2) | Insurance investment contract
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	80,263,000	[3]	77,044,000	[4]
Other Postretirement Benefits | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Other Postretirement Benefits | 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
Other Postretirement Benefits | 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
Other Postretirement Benefits | 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
Other Postretirement Benefits | Significant Unobservable Inputs (Level 3) | Insurance investment contract
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	$ 0	[3]	$ 0	[4]

[1] Collective and mutual funds invest approximately 29 percent in common stock of international companies, 21 percent in corporate bonds, 20 percent in common stock of large-cap U.S. companies, 8 percent in cash equivalents, 7 percent in U.S. Government securities and 15 percent in other investments.

[2] Collective and mutual funds invest approximately 29 percent in common stock of international companies, 19 percent in common stock of large-cap U.S. companies, 16 percent in corporate bonds, 16 percent in cash equivalents, 6 percent in common stock of mid-cap U.S. companies and 14 percent in other investments.

[3] 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.

[4] The insurance contract invests approximately 36 percent in corporate bonds, 22 percent in U.S. Government securities, 19 percent in common stock of large-cap U.S. companies, 10 percent in mortgage-backed securities and 13 percent in other investments.