Capital Stock - Stock Repurchases (Details) - Class A common stock - Stock repurchase program - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2011
Stock repurchases
Amount of authorized stock repurchases approved for continuation under the current program				$ 150,000,000
Shares repurchased under stock repurchase program (in shares)	0	0	0
Approximate amount of repurchase authority remaining under the current stock repurchase program	$ 17,800,000