Fair value measurements (Details 3) - USD ($) $ in Thousands	Dec. 31, 2017		Dec. 31, 2016		Sep. 30, 2015	Jun. 30, 2015
Concentration risks, percentage [Abstract]
Percentage in fixed-income investments	49.00%		52.00%
Percentage investment in common stock of large-cap companies	23.00%		22.00%
Percentage investment in common stock of mid-cap companies	14.00%		13.00%
Percentage investment in common stock of small-cap companies	11.00%		10.00%
Percentage investment in target date investments	2.00%		1.00%
Percentage investment in cash and cash equivalents	1.00%		2.00%
Fair value, measurements, recurring [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 94,774		$ 82,972
Fair value, measurements, recurring [Member] | Money market funds
Fair value measurements [Line Items]
Assets, fair value disclosure	6,965		1,602
Fair value, measurements, recurring [Member] | Insurance contract
Fair value measurements [Line Items]
Assets, fair value disclosure	77,388	[1]	70,921	[2]
Fair value, measurements, recurring [Member] | Mortgage-backed securities
Fair value measurements [Line Items]
Assets, fair value disclosure	10,217		10,449
Fair value, measurements, recurring [Member] | U.S. Treasury securities
Fair value measurements [Line Items]
Assets, fair value disclosure	204
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Money market funds
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Insurance contract
Fair value measurements [Line Items]
Assets, fair value disclosure	0	[1]	0	[2]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Mortgage-backed securities
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | U.S. Treasury securities
Fair value measurements [Line Items]
Assets, fair value disclosure	0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	94,774		82,972
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Money market funds
Fair value measurements [Line Items]
Assets, fair value disclosure	6,965		1,602
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Insurance contract
Fair value measurements [Line Items]
Assets, fair value disclosure	77,388	[1]	70,921	[2]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Mortgage-backed securities
Fair value measurements [Line Items]
Assets, fair value disclosure	10,217		10,449
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | U.S. Treasury securities
Fair value measurements [Line Items]
Assets, fair value disclosure	204
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Money market funds
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Insurance contract
Fair value measurements [Line Items]
Assets, fair value disclosure	0	[1]	0	[2]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Mortgage-backed securities
Fair value measurements [Line Items]
Assets, fair value disclosure	0		$ 0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | U.S. Treasury securities
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 0
Pipeline and midstream:
Fair value measurements [Line Items]
Nonrecurring fair value measurement					$ 10,800	$ 1,100

[1] 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.

[2] The insurance contract invests approximately 52 percent in fixed-income investments, 22 percent in common stock of large-cap companies, 13 percent in common stock of mid-cap companies, 10 percent in common stock of small-cap companies, 1 percent in target date investments and 2 percent in cash equivalents.