Available-for-sale securities (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Available-for-sale securities [Line Items]
Investments used to satisfy obligations under nonqualified benefit plans	$ 63,300,000	$ 53,300,000	$ 62,400,000
Net unrealized gain (loss) of investments used to satify obligations under nonqualified benefit plans	900,000	4,400,000
Available-for-sale securities [Abstract]
Cost	10,012,000	10,050,000	10,057,000
Gross unrealized gains	72,000	172,000	84,000
Gross unrealized losses	(17,000)	(3,000)	(31,000)
Fair value	10,067,000	10,219,000	10,110,000
Mortgage backed securities [Member]
Available-for-sale securities [Abstract]
Cost	7,943,000	8,749,000	8,151,000
Gross unrealized gains	63,000	133,000	69,000
Gross unrealized losses	(17,000)	(3,000)	(27,000)
Fair value	7,989,000	8,879,000	8,193,000
US Treasury securities [Member]
Available-for-sale securities [Abstract]
Cost	2,069,000	1,301,000	1,906,000
Gross unrealized gains	9,000	39,000	15,000
Gross unrealized losses	0	0	(4,000)
Fair value	$ 2,078,000	$ 1,340,000	$ 1,917,000