Income Taxes - Summary of reconciliation between federal statutory rate and effective tax rate (Details)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Taxes
Federal statutory rate	35.00%	35.00%	35.00%
State income taxes, net of federal benefit	3.20%	3.10%	2.90%
Tax credits	(0.30%)	(0.20%)	(0.60%)
Non-deductible expenses	0.50%	0.40%	0.40%
Other	(0.40%)	(0.20%)	0.50%
Effective tax rate	38.00%	38.10%	38.20%