Available-for-sale securities (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Available-for-sale securities [Line Items]
Investments used to satisfy obligations under nonqualified benefit plans	$ 67,800,000	$ 63,300,000	$ 65,800,000
Net unrealized gain (loss) of investments used to satify obligations under nonqualified benefit plans	2,000,000	900,000
Available-for-sale securities [Abstract]
Cost	10,129,000	10,012,000	10,168,000
Gross unrealized gains	67,000	72,000	60,000
Gross unrealized losses	(22,000)	(17,000)	(37,000)
Fair value	10,174,000	10,067,000	10,191,000
Mortgage backed securities [Member]
Available-for-sale securities [Abstract]
Cost	7,792,000	7,943,000	6,594,000
Gross unrealized gains	58,000	63,000	60,000
Gross unrealized losses	(18,000)	(17,000)	(18,000)
Fair value	7,832,000	7,989,000	6,636,000
US Treasury securities [Member]
Available-for-sale securities [Abstract]
Cost	2,337,000	2,069,000	3,574,000
Gross unrealized gains	9,000	9,000	0
Gross unrealized losses	(4,000)	0	(19,000)
Fair value	$ 2,342,000	$ 2,078,000	$ 3,555,000