Capital Stock - Class A and B Common Stock (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Common stock
Number of classes of common stock	2
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 (as a percent)	0.67%
Ratio for converting shares of Class B common stock into shares of Class A common stock (as a percent)	2400.00%	2400.00%
Class B common stock shares converted into Class A common stock shares (in shares)	0	0	0
Class A Common Stock
Common stock
Minimum multiplier for dividend payable on Class A common stock per dividend declared on Class B common stock (as a percent)	0.67%