Capital Stock - Class A and B Common Stock (Details)	12 Months Ended
	Dec. 31, 2020 classOfStock shares	Dec. 31, 2019 shares	Dec. 31, 2018 shares
Common stock
Number of classes of common stock | classOfStock	2
Class A common stock
Common stock
Minimum multiplier for dividend payable on Class A common stock per dividend declared on Class B common stock	0.67%
Class B common stock
Common stock
Percentage of dividend declared on Class B common stock used to determine dividend payable on Class A common stock	0.67%
Ratio for converting shares of Class B common stock into shares of Class A common stock	2,400	2,400
Class B common stock shares converted into Class A common stock shares (in shares) | shares	0	0	0