Fair value measurements (Details 2) - USD ($) $ in Thousands	Jun. 30, 2015		Dec. 31, 2014		Jun. 30, 2014
Concentration risks, percentage [Abstract]
Percentage investment in common stock of mid-cap companies	20.00%		20.00%		21.00%
Percentage investment in common stock of small-cap companies	18.00%		18.00%		18.00%
Percentage investment in common stock of large-cap companies	28.00%		29.00%		29.00%
Percentage in fixed-income and other investments	32.00%		32.00%		31.00%
Percentage Investment in Target Date Investments	1.00%
Percentage investment in cash and cash equivalents	1.00%		1.00%		1.00%
Assets, Fair Value Disclosure, Nonrecurring	$ 1,100
Fair value, measurements, recurring [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	94,967		$ 92,160		$ 90,651
Liabilities, fair value disclosure	538
Fair value, measurements, recurring [Member] | Renewable Identification Number Obligations [Member]
Fair value measurements [Line Items]
Liabilities, fair value disclosure	538
Fair value, measurements, recurring [Member] | Money market funds [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	16,358		16,138		16,031
Fair value, measurements, recurring [Member] | Insurance contract [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	68,187	[1]	65,831	[2]	64,449	[3]
Fair value, measurements, recurring [Member] | Mortgage backed securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	8,073		6,636		8,075
Fair value, measurements, recurring [Member] | US Treasury securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	2,349		3,555		2,096
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Liabilities, fair value disclosure	0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Renewable Identification Number Obligations [Member]
Fair value measurements [Line Items]
Liabilities, fair value disclosure	0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Money market funds [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Insurance contract [Member]
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 [Member] | Mortgage backed securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | US Treasury securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	94,967		92,160		90,651
Liabilities, fair value disclosure	538
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Renewable Identification Number Obligations [Member]
Fair value measurements [Line Items]
Liabilities, fair value disclosure	538
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Money market funds [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	16,358		16,138		16,031
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Insurance contract [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	68,187	[1]	65,831	[2]	64,449	[3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Mortgage backed securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	8,073		6,636		8,075
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | US Treasury securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	2,349		3,555		2,096
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Liabilities, fair value disclosure	0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Renewable Identification Number Obligations [Member]
Fair value measurements [Line Items]
Liabilities, fair value disclosure	0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Money market funds [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Insurance contract [Member]
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 [Member] | Mortgage backed securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | US Treasury securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 0		$ 0		$ 0

[1] The insurance contract invests approximately 20 percent in common stock of mid-cap companies, 18 percent in common stock of small-cap companies, 28 percent in common stock of large-cap companies, 32 percent in fixed-income investments, 1 percent in target date investments and 1 percent in cash equivalents.

[2] The insurance contract invests approximately 20 percent in common stock of mid-cap companies, 18 percent in common stock of small-cap companies, 29 percent in common stock of large-cap companies, 32 percent in fixed-income investments and 1 percent in cash equivalents.

[3] The insurance contract invests approximately 21 percent in common stock of mid-cap companies, 18 percent in common stock of small-cap companies, 29 percent in common stock of large-cap companies, 31 percent in fixed-income investments and 1 percent in cash equivalents.