Available-for-sale securities (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Available-for-sale securities [Line Items]
Investments used to satisfy obligations under nonqualified benefit plans	$ 62,400,000	$ 48,900,000
Net unrealized gain (loss) of investments used to satify obligations under nonqualified benefit plans	13,500,000	5,200,000	(1,100,000)
Available-for-sale securities:
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	10,057,000	9,817,000
Available-for-sale securities, gross unrealized gains	84,000	187,000
Available-for-sale securities, gross unrealized losses	(31,000)	(3,000)
Available-for-sale securities	10,110,000	10,001,000
Mortgage-backed securities
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	8,151,000	8,054,000
Available-for-sale securities, gross unrealized gains	69,000	144,000
Available-for-sale securities, gross unrealized losses	(27,000)	(3,000)
Available-for-sale securities	8,193,000	8,195,000
U.S. Treasury securities
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	1,906,000	1,763,000
Available-for-sale securities, gross unrealized gains	15,000	43,000
Available-for-sale securities, gross unrealized losses	(4,000)	0
Available-for-sale securities	$ 1,917,000	$ 1,806,000