Available-for-sale securities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Available-for-sale securities [Line Items]
Investments used to satisfy obligations under nonqualified benefit plans	$ 69,100	$ 67,800	$ 67,500
Net unrealized gain (loss) of investments used to satify obligations under nonqualified benefit plans	1,600	2,000
Available-for-sale securities [Abstract]
Cost	10,467	10,129	10,443
Gross unrealized gains	46	67	19
Gross unrealized losses	(14)	(22)	(55)
Fair value	10,499	10,174	10,407
Mortgage backed securities [Member]
Available-for-sale securities [Abstract]
Cost	10,467	7,792	9,128
Gross unrealized gains	46	58	19
Gross unrealized losses	(14)	(18)	(49)
Fair value	$ 10,499	7,832	9,098
US Treasury securities [Member]
Available-for-sale securities [Abstract]
Cost		2,337	1,315
Gross unrealized gains		9	0
Gross unrealized losses		(4)	(6)
Fair value		$ 2,342	$ 1,309