Available-for-sale securities (Details) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Available-for-sale securities [Line Items]
Investments used to satisfy obligations under nonqualified benefit plans	$ 64,400,000	$ 54,000,000	$ 64,400,000	$ 54,000,000	$ 62,400,000
Net unrealized gain (loss) of investments used to satify obligations under nonqualified benefit plans	1,100,000	700,000	2,000,000	5,100,000
Available-for-sale securities [Abstract]
Cost	10,055,000	9,955,000	10,055,000	9,955,000	10,057,000
Gross unrealized gains			121,000	73,000	84,000
Gross unrealized losses			(5,000)	(56,000)	(31,000)
Fair value	10,171,000	9,972,000	10,171,000	9,972,000	10,110,000
Mortgage backed securities [Member]
Available-for-sale securities [Abstract]
Cost	7,989,000	8,035,000	7,989,000	8,035,000	8,151,000
Gross unrealized gains			91,000	58,000	69,000
Gross unrealized losses			(5,000)	(41,000)	(27,000)
Fair value	8,075,000	8,052,000	8,075,000	8,052,000	8,193,000
US Treasury securities [Member]
Available-for-sale securities [Abstract]
Cost	2,066,000	1,920,000	2,066,000	1,920,000	1,906,000
Gross unrealized gains			30,000	15,000	15,000
Gross unrealized losses			0	(15,000)	(4,000)
Fair value	$ 2,096,000	$ 1,920,000	$ 2,096,000	$ 1,920,000	$ 1,917,000