Fair value measurements (Details 3) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Concentration risks, percentage [Abstract]
Percentage in fixed-income and other investments	47.00%	49.00%	50.00%
Percentage investment in common stock of large-cap companies	24.00%	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	12.00%	11.00%	11.00%
Percentage investment in target date investments	3.00%	2.00%	2.00%
Percentage investment in cash and cash equivalents	1.00%	1.00%	1.00%
Fair value, measurements, recurring [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 100,787	$ 94,774	$ 91,234
Fair value, measurements, recurring [Member] | Money market funds
Fair value measurements [Line Items]
Assets, fair value disclosure	9,948	6,965	6,204
Fair value, measurements, recurring [Member] | Insurance contract
Fair value measurements [Line Items]
Assets, fair value disclosure	80,358	77,388	74,991
Fair value, measurements, recurring [Member] | Mortgage backed securities
Fair value measurements [Line Items]
Assets, fair value disclosure	10,302	10,217	9,427
Fair value, measurements, recurring [Member] | US Treasury securities
Fair value measurements [Line Items]
Assets, fair value disclosure	179	204	612
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	0	0
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	100,787	94,774	91,234
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | Money market funds
Fair value measurements [Line Items]
Assets, fair value disclosure	9,948	6,965	6,204
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | Insurance contract
Fair value measurements [Line Items]
Assets, fair value disclosure	80,358	77,388	74,991
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | Mortgage backed securities
Fair value measurements [Line Items]
Assets, fair value disclosure	10,302	10,217	9,427
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | US Treasury securities
Fair value measurements [Line Items]
Assets, fair value disclosure	179	204	612
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	0	0
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