Stock awards (Details 3) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Stock compensation plan [Member]
Share-based compensation arrangement by share-based payment award [Line Items]
Nonemployee director stock compensation plan, number of shares granted	53,888	55,141	43,128
Nonemployee director stock compensation plan, fair value of shares granted	$ 1,100,000	$ 1,100,000	$ 849,000
Long-term incentive plan [Member]
Share-based compensation arrangement by share-based payment award [Line Items]
Common shares granted under a long-term incentive plan	43,103
Fair value of common stock granted under a long-term incentive plan	$ 930,000