Fair value measurements (Details 2) (USD $)	Dec. 31, 2014		Dec. 31, 2013		Jun. 30, 2013
Concentration risks, percentage [Abstract]
Percentage investment in common stock of mid-cap companies	20.00%		29.00%
Percentage investment in common stock of small-cap companies	18.00%		28.00%
Percentage investment in common stock of large-cap companies	29.00%		28.00%
Percentage in fixed-income investments	32.00%		15.00%
Percentage investment in cash and cash equivalents	1.00%
Assets, fair value disclosure, nonrecurring					$ 9,700,000
Fair value, measurements, recurring [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	112,830,000		93,657,000
Liabilities:			7,483,000
Fair value, measurements, recurring [Member] | Commodity derivative instruments
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Liabilities:			7,483,000
Fair value, measurements, recurring [Member] | Money market funds
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	18,473,000		19,227,000
Fair value, measurements, recurring [Member] | Insurance contract [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	65,831,000	[1]	62,370,000	[2]
Fair value, measurements, recurring [Member] | Mortgage-backed securities
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	6,636,000		8,193,000
Fair value, measurements, recurring [Member] | U.S. Government securities [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	3,555,000		1,917,000
Fair value, measurements, recurring [Member] | Commodity derivative instruments
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	18,335,000		1,950,000
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	0		0
Liabilities:			0
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Commodity derivative instruments
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Liabilities:			0
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	0		0
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Insurance contract [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	0	[1]	0	[2]
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mortgage-backed securities
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	0		0
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Government securities [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	0		0
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Commodity derivative instruments
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	0		0
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2)
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	112,830,000		93,657,000
Liabilities:			7,483,000
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Commodity derivative instruments
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Liabilities:			7,483,000
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Money market funds
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	18,473,000		19,227,000
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Insurance contract [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	65,831,000	[1]	62,370,000	[2]
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	6,636,000		8,193,000
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | U.S. Government securities [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	3,555,000		1,917,000
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Commodity derivative instruments
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	18,335,000		1,950,000
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3)
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	0		0
Liabilities:			0
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | Commodity derivative instruments
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Liabilities:			0
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | Money market funds
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	0		0
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | Insurance contract [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	0	[1]	0	[2]
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | Mortgage-backed securities
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	0		0
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | U.S. Government securities [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	0		0
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | Commodity derivative instruments
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	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, 29 percent in common stock of large-cap companies, 32 percent in fixed-income investments and 1 percent in cash equivalents.

[2] 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 investments.