Segment information (Details Textual) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Segment information (Textual) [Abstract]
Taxable equivalent adjustment	$ 6,645,000	$ 6,468,000	$ 13,350,000	$ 12,795,000