Stock-based compensation - Director deferred compensation (Details) (Director)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Director
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Beginning balance, Directors deferred compensation account (in shares)	129,264
New shares deferred (in shares)	13,636
Issued (in shares)	(7,178)	(12,909)	(6,592)
Ending balance, Directors deferred compensation account (in shares)	135,722	129,264