Fair value measurements (Details 2) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014		Mar. 31, 2014
Concentration risks, percentage [Abstract]
Percentage investment in common stock of mid-cap companies	20.00%		20.00%		29.00%
Percentage investment in common stock of small-cap companies	18.00%		18.00%		27.00%
Percentage investment in common stock of large-cap companies	28.00%		29.00%		28.00%
Percentage in fixed-income and other investments	32.00%		32.00%		16.00%
Percentage Investment in Target Date Investments	1.00%
Percentage investment in cash and cash equivalents	1.00%		1.00%
Fair value, measurements, recurring [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	102,043		112,830		93,933
Liabilities, fair value disclosure					12,186
Fair value, measurements, recurring [Member] | Money market funds [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	16,945		18,473		20,267
Fair value, measurements, recurring [Member] | Insurance contract [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	67,797	[1]	65,831	[2]	63,269	[3]
Fair value, measurements, recurring [Member] | Mortgage backed securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	7,832		6,636		7,989
Fair value, measurements, recurring [Member] | US Treasury securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	2,342		3,555		2,078
Fair value, measurements, recurring [Member] | Commodity derivative instruments , assets [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	7,127		18,335		330
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] | 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 1 [Member] | Commodity derivative instruments , assets [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	102,043		112,830		93,933
Liabilities, fair value disclosure					12,186
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Money market funds [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	16,945		18,473		20,267
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Insurance contract [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	67,797	[1]	65,831	[2]	63,269	[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	7,832		6,636		7,989
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | US Treasury securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	2,342		3,555		2,078
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Commodity derivative instruments , assets [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	7,127		18,335		330
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] | 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
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Commodity derivative instruments , assets [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	0		0		0
Derivative Financial Instruments, Liabilities [Member] | Fair value, measurements, recurring [Member]
Fair value measurements [Line Items]
Liabilities, fair value disclosure					12,186
Derivative Financial Instruments, Liabilities [Member] | Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member]
Fair value measurements [Line Items]
Liabilities, fair value disclosure					0
Derivative Financial Instruments, Liabilities [Member] | Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member]
Fair value measurements [Line Items]
Liabilities, fair value disclosure					12,186
Derivative Financial Instruments, Liabilities [Member] | Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member]
Fair value measurements [Line Items]
Liabilities, fair value disclosure					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 29 percent in common stock of mid-cap companies, 27 percent in common stock of small-cap companies, 28 percent in common stock of large-cap companies and 16 percent in fixed-income investments.