Income Taxes - Income tax rates (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Effective income tax rate differs from the statutory rate
Effective income tax rate (as a percent)	27.40%	33.00%	28.30%	31.90%