Available-for-sale securities (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Available-for-sale securities [Line Items]
Investments used to satisfy obligations under nonqualified benefit plans	$ 71,400,000	$ 68,200,000	$ 71,400,000	$ 68,200,000	$ 67,500,000
Net unrealized gain (loss) of investments used to satify obligations under nonqualified benefit plans	2,300,000	400,000	3,900,000	2,400,000
Available-for-sale securities [Abstract]
Cost	10,420,000	10,399,000	10,420,000	10,399,000	10,443,000
Gross unrealized gains	52,000	51,000	52,000	51,000	19,000
Gross unrealized losses	(12,000)	(28,000)	(12,000)	(28,000)	(55,000)
Fair value	10,460,000	10,422,000	10,460,000	10,422,000	10,407,000
Mortgage backed securities [Member]
Available-for-sale securities [Abstract]
Cost	10,420,000	8,072,000	10,420,000	8,072,000	9,128,000
Gross unrealized gains	52,000	29,000	52,000	29,000	19,000
Gross unrealized losses	(12,000)	(28,000)	(12,000)	(28,000)	(49,000)
Fair value	$ 10,460,000	8,073,000	$ 10,460,000	8,073,000	9,098,000
US Treasury securities [Member]
Available-for-sale securities [Abstract]
Cost		2,327,000		2,327,000	1,315,000
Gross unrealized gains		22,000		22,000	0
Gross unrealized losses		0		0	(6,000)
Fair value		$ 2,349,000		$ 2,349,000	$ 1,309,000