Fair value measurements (Details 3) - USD ($) $ in Thousands	Jun. 30, 2018		Dec. 31, 2017		Jun. 30, 2017
Concentration risks, percentage [Abstract]
Percentage in fixed-income and other investments	48.00%		49.00%		50.00%
Percentage investment in common stock of large-cap companies	23.00%		23.00%		23.00%
Percentage investment in common stock of mid-cap companies	13.00%		14.00%		13.00%
Percentage investment in common stock of small-cap companies	11.00%		11.00%		11.00%
Percentage investment in target date investments	3.00%		2.00%		2.00%
Percentage investment in cash and cash equivalents	2.00%		1.00%		1.00%
Fair value, measurements, recurring [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 98,241		$ 94,774		$ 89,291
Fair value, measurements, recurring [Member] | Money market funds
Fair value measurements [Line Items]
Assets, fair value disclosure	9,904		6,965		5,882
Fair value, measurements, recurring [Member] | Insurance contract
Fair value measurements [Line Items]
Assets, fair value disclosure	78,312	[1]	77,388	[2]	73,126	[3]
Fair value, measurements, recurring [Member] | Mortgage backed securities
Fair value measurements [Line Items]
Assets, fair value disclosure	9,785		10,217		9,670
Fair value, measurements, recurring [Member] | US Treasury securities
Fair value measurements [Line Items]
Assets, fair value disclosure	240		204		613
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 | Money market funds
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 | Insurance contract
Fair value measurements [Line Items]
Assets, fair value disclosure	0	[1]	0	[2]	0	[3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 | Mortgage backed securities
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 | US Treasury securities
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2
Fair value measurements [Line Items]
Assets, fair value disclosure	98,241		94,774		89,291
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | Money market funds
Fair value measurements [Line Items]
Assets, fair value disclosure	9,904		6,965		5,882
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | Insurance contract
Fair value measurements [Line Items]
Assets, fair value disclosure	78,312	[1]	77,388	[2]	73,126	[3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | Mortgage backed securities
Fair value measurements [Line Items]
Assets, fair value disclosure	9,785		10,217		9,670
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | US Treasury securities
Fair value measurements [Line Items]
Assets, fair value disclosure	240		204		613
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 | Money market funds
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 | Insurance contract
Fair value measurements [Line Items]
Assets, fair value disclosure	0	[1]	0	[2]	0	[3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 | Mortgage backed securities
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 | US Treasury securities
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 0		$ 0		$ 0

[1] The insurance contract invests approximately 48 percent in fixed-income investments, 23 percent in common stock of large-cap companies, 13 percent in common stock of mid-cap companies, 11 percent in common stock of small-cap companies, 3 percent in target date investments and 2 percent in cash equivalents.

[2] The insurance contract invests approximately 49 percent in fixed-income investments, 23 percent in common stock of large-cap companies, 14 percent in common stock of mid-cap companies, 11 percent in common stock of small-cap companies, 2 percent in target date investments and 1 percent in cash equivalents.

[3] The insurance contract invests approximately 50 percent in fixed-income investments, 23 percent in common stock of large-cap companies, 13 percent in common stock of mid-cap companies, 11 percent in common stock of small-cap companies, 2 percent in target date investments and 1 percent in cash equivalents.