Income Taxes - Summary of reconciliation between federal statutory rate and effective tax rate (Details 1)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Taxes
Federal statutory rate	35.00%	35.00%	35.00%
State income taxes, net of federal benefit		3.30%	3.80%
Tax credits	0.30%	(0.70%)	(0.30%)
Non-deductible expenses	(1.30%)	0.40%	0.50%
Other	0.90%	0.60%	0.60%
Effective tax rate	34.90%	38.60%	39.60%