Available-for-sale securities (Details) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Available-for-sale securities [Line Items]
Investments used to satisfy obligations under nonqualified benefit plans	$ 58,100,000	$ 48,400,000	$ 58,100,000	$ 48,400,000	$ 48,900,000
Net unrealized gain (loss) of investments used to satify obligations under nonqualified benefit plans	4,100,000	2,400,000	9,200,000	4,700,000
Available-for-sale securities [Abstract]
Cost	9,963,000	10,149,000	9,963,000	10,149,000	9,817,000
Gross unrealized gains			85,000	222,000	187,000
Gross unrealized losses			(24,000)	(2,000)	(3,000)
Fair value	10,024,000	10,369,000	10,024,000	10,369,000	10,001,000
Mortgage backed securities [Member]
Available-for-sale securities [Abstract]
Cost	8,051,000	8,391,000	8,051,000	8,391,000	8,054,000
Gross unrealized gains			70,000	175,000	144,000
Gross unrealized losses			(20,000)	(2,000)	(3,000)
Fair value	8,101,000	8,564,000	8,101,000	8,564,000	8,195,000
US Treasury securities [Member]
Available-for-sale securities [Abstract]
Cost	1,912,000	1,758,000	1,912,000	1,758,000	1,763,000
Gross unrealized gains			15,000	47,000	43,000
Gross unrealized losses			(4,000)	0	0
Fair value	$ 1,923,000	$ 1,805,000	$ 1,923,000	$ 1,805,000	$ 1,806,000