Fair value measurements (Details 2) (USD $)	Sep. 30, 2013		Jun. 30, 2013	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012
Concentration risks, percentage [Abstract]
Percentage investment in common stock of mid-cap companies	29.00%			28.00%		28.00%
Percentage investment in common stock of small-cap companies	28.00%			28.00%		28.00%
Percentage investment in common stock of large-cap companies	28.00%			29.00%		29.00%
Percentage in fixed-income and other investments	15.00%			15.00%		15.00%
Assets, fair value disclosure, nonrecurring			$ 9,700,000					$ 2,500,000
Fair value, measurements, recurring [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	95,329,000			101,443,000		103,288,000
Liabilities, fair value disclosure	9,889,000			6,255,000		9,934,000
Fair value, measurements, recurring [Member] | Commodity derivative instruments [Member]
Fair value measurements [Line Items]
Liabilities, fair value disclosure	9,889,000					2,155,000
Fair value, measurements, recurring [Member] | Interest rate swap [Member]
Fair value measurements [Line Items]
Liabilities, fair value disclosure				6,255,000		7,779,000
Fair value, measurements, recurring [Member] | Money market funds [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	21,019,000			24,240,000		21,816,000
Fair value, measurements, recurring [Member] | Insurance contract [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	58,142,000	[1]		48,898,000	[2]	48,384,000	[2]
Fair value, measurements, recurring [Member] | Mortgage backed securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	8,101,000			8,195,000		8,564,000
Fair value, measurements, recurring [Member] | US Treasury securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	1,923,000			1,806,000		1,805,000
Fair value, measurements, recurring [Member] | Commodity derivative instruments , assets [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	6,144,000			18,304,000		22,719,000
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			0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Commodity derivative instruments [Member]
Fair value measurements [Line Items]
Liabilities, fair value disclosure	0					0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Interest rate swap [Member]
Fair value measurements [Line Items]
Liabilities, fair value disclosure				0		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	[2]
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	95,329,000			101,443,000		103,288,000
Liabilities, fair value disclosure	9,889,000			6,255,000		9,934,000
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Commodity derivative instruments [Member]
Fair value measurements [Line Items]
Liabilities, fair value disclosure	9,889,000					2,155,000
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Interest rate swap [Member]
Fair value measurements [Line Items]
Liabilities, fair value disclosure				6,255,000		7,779,000
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Money market funds [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	21,019,000			24,240,000		21,816,000
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Insurance contract [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	58,142,000	[1]		48,898,000	[2]	48,384,000	[2]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Mortgage backed securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	8,101,000			8,195,000		8,564,000
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | US Treasury securities [Member]
Fair value measurements [Line Items]
Assets, fair value disclosure	1,923,000			1,806,000		1,805,000
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	6,144,000			18,304,000		22,719,000
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			0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Commodity derivative instruments [Member]
Fair value measurements [Line Items]
Liabilities, fair value disclosure	0					0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Interest rate swap [Member]
Fair value measurements [Line Items]
Liabilities, fair value disclosure				0		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	[2]
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

[1] The insurance contract invests approximately 29 percent in common stock of mid-cap companies, 28 percent in common stock of small-cap companies, 28 percent in common stock of large-cap companies and 15 percent in fixed-income and other investments.

[2] The insurance contract invests approximately 28 percent in common stock of mid-cap companies, 28 percent in common stock of small-cap companies, 29 percent in common stock of large-cap companies and 15 percent in fixed-income and other investments.