Fair value measurements (Details 2) - USD ($) $ in Thousands	Sep. 30, 2015		Jun. 30, 2015	Dec. 31, 2014		Sep. 30, 2014
Concentration risks, percentage [Abstract]
Percentage investment in common stock of mid-cap companies	9.00%			20.00%		21.00%
Percentage investment in common stock of small-cap companies	6.00%			18.00%		17.00%
Percentage investment in common stock of large-cap companies	18.00%			29.00%		29.00%
Percentage in fixed-income and other investments	65.00%			32.00%		32.00%
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	$ 10,800		$ 1,100
Fair value, measurements, recurring [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	77,861			$ 76,912		$ 74,537
Liabilities, fair value disclosure	1,170
Fair value, measurements, recurring [Member] | Renewable Identification Number Obligations [Member]
Fair value measurements [Line Items]
Liabilities, fair value disclosure	1,170
Fair value, measurements, recurring [Member] | Money market funds [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	1,219			890		684
Fair value, measurements, recurring [Member] | Insurance contract [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	66,464	[1]		65,831	[2]	63,578	[3]
Fair value, measurements, recurring [Member] | Mortgage backed securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	7,854			6,636		7,901
Fair value, measurements, recurring [Member] | US Treasury securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	2,324			3,555		2,374
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	77,861			76,912		74,537
Liabilities, fair value disclosure	1,170
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	1,170
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Money market funds [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	1,219			890		684
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Insurance contract [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	66,464	[1]		65,831	[2]	63,578	[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	7,854			6,636		7,901
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | US Treasury securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	2,324			3,555		2,374
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 9 percent in common stock of mid-cap companies, 6 percent in common stock of small-cap companies, 18 percent in common stock of large-cap companies, 65 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, 17 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.