Fair value measurements (Details 3) - USD ($) $ in Thousands	Dec. 31, 2016		Dec. 31, 2015		Sep. 30, 2015	Jun. 30, 2015
Fair value measurements [Line Items]
Nonrecurring fair value measurement					$ 10,800	$ 1,100
Concentration risks, percentage [Abstract]
Percentage in fixed-income investments	52.00%		63.00%
Percentage investment in common stock of large-cap companies	22.00%		19.00%
Percentage investment in common stock of mid-cap companies	13.00%		9.00%
Percentage investment in common stock of small-cap companies	10.00%		7.00%
Percentage investment in target date investments	1.00%		1.00%
Percentage investment in cash and cash equivalents	2.00%		1.00%
Fair value, measurements, recurring [Member]
Fair value measurements [Line Items]
Assets:	$ 82,972		$ 79,286
Fair value, measurements, recurring [Member] | Money market funds
Fair value measurements [Line Items]
Assets:	1,602		1,420
Fair value, measurements, recurring [Member] | Insurance contract
Fair value measurements [Line Items]
Assets:	70,921	[1]	67,459	[2]
Fair value, measurements, recurring [Member] | Mortgage-backed securities
Fair value measurements [Line Items]
Assets:	10,449		9,098
Fair value, measurements, recurring [Member] | U.S. Treasury securities
Fair value measurements [Line Items]
Assets:			1,309
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value measurements [Line Items]
Assets:	0		0
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds
Fair value measurements [Line Items]
Assets:	0		0
Fair value, measurements, recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Insurance contract
Fair value measurements [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 measurements [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 measurements [Line Items]
Assets:			0
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2)
Fair value measurements [Line Items]
Assets:	82,972		79,286
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Money market funds
Fair value measurements [Line Items]
Assets:	1,602		1,420
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Insurance contract
Fair value measurements [Line Items]
Assets:	70,921	[1]	67,459	[2]
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
Fair value measurements [Line Items]
Assets:	10,449		9,098
Fair value, measurements, recurring [Member] | Significant Other Observable Inputs (Level 2) | U.S. Treasury securities
Fair value measurements [Line Items]
Assets:			1,309
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3)
Fair value measurements [Line Items]
Assets:	0		0
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | Money market funds
Fair value measurements [Line Items]
Assets:	0		0
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | Insurance contract
Fair value measurements [Line Items]
Assets:	0	[1]	0	[2]
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | Mortgage-backed securities
Fair value measurements [Line Items]
Assets:	$ 0		0
Fair value, measurements, recurring [Member] | Significant Unobservable Inputs (Level 3) | U.S. Treasury securities
Fair value measurements [Line Items]
Assets:			$ 0

[1] The insurance contract invests approximately 52 percent in fixed-income investments, 22 percent in common stock of large-cap companies, 13 percent in common stock of mid-cap companies, 10 percent in common stock of small-cap companies, 1 percent in target date investments and 2 percent in cash equivalents.

[2] The insurance contract invests approximately 63 percent in fixed-income investments, 19 percent in common stock of large-cap companies, 9 percent in common stock of mid-cap companies, 7 percent in common stock of small-cap companies, 1 percent in target date investments and 1 percent in cash equivalents.