Capital Stock (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Aggregate Value of Shares Issued	$ 0.5
Class A Common Stock
Class of Stock [Line Items]
Common stock outstanding (in shares)	34,200,792	33,560,202
Stock repurchased and retired (in shares)	0
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period	11,389
Class B Common Stock
Class of Stock [Line Items]
Common stock outstanding (in shares)	22,559,402	22,905,357