Available-for-sale securities (Details) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Schedule of available-for-sale securities [Line Items]
Investments used to satisfy obligations under nonqualified benefit plans	$ 54,000,000	$ 46,000,000	$ 54,000,000	$ 46,000,000	$ 48,900,000
Net unrealized gain (loss) of investments used to satify obligations under nonqualified benefit plans	700,000	(2,700,000)	5,100,000	2,200,000
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	47,225,000	47,338,000	47,225,000	47,338,000	47,067,000
Available-for-sale securities, gross unrealized gains			16,842,000	8,874,000	11,835,000
Available-for-sale securities, gross unrealized losses			(56,000)	(6,000)	(3,000)
Available-for-sale securities	64,011,000	56,206,000	64,011,000	56,206,000	58,899,000
Insurance contract [Member]
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	37,270,000	37,250,000	37,270,000	37,250,000	37,250,000
Available-for-sale securities, gross unrealized gains			16,769,000	8,709,000	11,648,000
Available-for-sale securities, gross unrealized losses			0	0	0
Available-for-sale securities	54,039,000	45,959,000	54,039,000	45,959,000	48,898,000
Collateralized mortgage backed securities [Member]
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	8,035,000	8,130,000	8,035,000	8,130,000	8,054,000
Available-for-sale securities, gross unrealized gains			58,000	128,000	144,000
Available-for-sale securities, gross unrealized losses			(41,000)	(5,000)	(3,000)
Available-for-sale securities	8,052,000	8,253,000	8,052,000	8,253,000	8,195,000
US Treasury securities [Member]
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	1,920,000	1,958,000	1,920,000	1,958,000	1,763,000
Available-for-sale securities, gross unrealized gains			15,000	37,000	43,000
Available-for-sale securities, gross unrealized losses			(15,000)	(1,000)	0
Available-for-sale securities	$ 1,920,000	$ 1,994,000	$ 1,920,000	$ 1,994,000	$ 1,806,000