Capital Stock (Details) - USD ($)	1 Months Ended	6 Months Ended	126 Months Ended
	Apr. 30, 2017	May 31, 2017	May 31, 2017
Equity, Class of Treasury Stock [Line Items]
Stock repurchased during the period		$ 15,992,000
Dividend declared (in dollars per share)	$ 0.43
Cash dividend paid ($0.43 per share)		19,249,000
Class A Common Stock | Stock Purchase Plan [Member]
Equity, Class of Treasury Stock [Line Items]
Authorized amount under Stock Purchase Plan		$ 530,000,000	$ 530,000,000
Stock repurchased during the period (shares)		438,328	9,160,401
Stock repurchased during the period		$ 16,000,000	$ 339,400,000
Weighted average price of stock repurchased (in US$ per share)		$ 36.48
Remaining repurchase authority under the Stock Purchase Plan		$ 190,600,000	$ 190,600,000