Fair value measurements (Details 2) - USD ($) $ in Thousands	Dec. 31, 2015		Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014
Concentration risks, percentage [Abstract]
Percentage investment in common stock of mid-cap companies	9.00%				20.00%
Percentage investment in common stock of small-cap companies	7.00%				18.00%
Percentage investment in common stock of large-cap companies	19.00%				29.00%
Percentage in fixed-income investments	63.00%				32.00%
Percentage Investment in Target Date Investments	1.00%
Percentage investment in cash and cash equivalents	1.00%				1.00%
Assets, fair value disclosure, nonrecurring			$ 10,800	$ 1,100
Fair value, measurements, recurring [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	$ 79,286				$ 76,912
Liabilities:	3,052
Fair value, measurements, recurring [Member] | Renewable Identification Number Obligations [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Liabilities:	3,052
Fair value, measurements, recurring [Member] | Money market funds
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	1,420				890
Fair value, measurements, recurring [Member] | Insurance contract [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	67,459	[1]			65,831	[2]
Fair value, measurements, recurring [Member] | Mortgage-backed securities
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	9,098				6,636
Fair value, measurements, recurring [Member] | U.S. Treasury securities
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	1,309				3,555
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) | Renewable Identification Number Obligations [Member]
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. Treasury securities
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:	79,286				76,912
Liabilities:	3,052
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Renewable Identification Number Obligations [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Liabilities:	3,052
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:	1,420				890
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:	67,459	[1]			65,831	[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:	9,098				6,636
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | U.S. Treasury securities
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	1,309				3,555
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) | Renewable Identification Number Obligations [Member]
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. Treasury securities
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets:	$ 0				$ 0

[1] The insurance contract invests approximately 9 percent in common stock of mid-cap companies, 7 percent in common stock of small-cap companies, 19 percent in common stock of large-cap companies, 63 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.