Stock-based expense and assumptions (Details) (USD $) Share data in Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 item	Jun. 30, 2012
Stock-based expense
Number of stock-based compensation programs			2
Stock-based expense
Total stock-based expense	$ 290,027	$ 316,005	$ 548,956	$ 540,397
Stock options
Stock-based expense
Options granted (in shares)	0.9
Service period			36 months
Total stock-based expense	252,640	316,005	511,569	540,397
Restricted stock awards
Stock-based expense
Service period			36 months
Stock granted (in shares)	0.7
Total stock-based expense	37,387		37,387
Processing and service costs
Stock-based expense
Total stock-based expense	58,162	48,220	107,805	129,001
Selling, general and administrative expenses
Stock-based expense
Total stock-based expense	$ 231,865	$ 267,785	$ 441,151	$ 411,396