Available-for-sale securities (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Available-for-sale securities [Line Items]
Investments used to satisfy obligations under nonqualified benefit plans	$ 66,500,000	$ 63,600,000	$ 66,500,000	$ 63,600,000	$ 65,800,000
Net unrealized gain (loss) of investments used to satify obligations under nonqualified benefit plans	(1,700,000)	(800,000)	700,000	1,200,000
Available-for-sale securities [Abstract]
Cost	10,167,000	10,206,000	10,167,000	10,206,000	10,168,000
Gross unrealized gains	33,000	79,000	33,000	79,000	60,000
Gross unrealized losses	(22,000)	(10,000)	(22,000)	(10,000)	(37,000)
Fair value	10,178,000	10,275,000	10,178,000	10,275,000	10,191,000
Mortgage backed securities [Member]
Available-for-sale securities [Abstract]
Cost	7,843,000	7,838,000	7,843,000	7,838,000	6,594,000
Gross unrealized gains	29,000	71,000	29,000	71,000	60,000
Gross unrealized losses	(18,000)	(8,000)	(18,000)	(8,000)	(18,000)
Fair value	7,854,000	7,901,000	7,854,000	7,901,000	6,636,000
US Treasury securities [Member]
Available-for-sale securities [Abstract]
Cost	2,324,000	2,368,000	2,324,000	2,368,000	3,574,000
Gross unrealized gains	4,000	8,000	4,000	8,000	0
Gross unrealized losses	(4,000)	(2,000)	(4,000)	(2,000)	(19,000)
Fair value	$ 2,324,000	$ 2,374,000	$ 2,324,000	$ 2,374,000	$ 3,555,000