Capital Stock (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Sep. 12, 2020	Jun. 11, 2019
Class of Stock [Line Items]
Stock Repurchase Program, Number of Shares Authorized to be Repurchased						5,500,000
Incremental Common Shares Attributable to Dilutive Effect of Accelerated Share Repurchase Agreements			3,000,000.0
Stock repurchased and retired (in shares)	1,445,300	4,200	2,539,041	43,560
Accelerated Share Repurchases, Adjustment to Recorded Amount	$ 46.3		$ 77.5
Shares repurchased, average cost per share (in dollars per share)	$ 32.05		$ 31.01
Shares repurchased during the period as percentage of shares authorized for repurchase (percent)	26.30%		45.50%
Class A Common Stock
Class of Stock [Line Items]
Shares outstanding (in shares)	41,143,592		41,143,592		43,129,085
Stock Repurchase Program, Number of Shares Authorized to be Repurchased							2,500,000
Stock repurchased and retired (in shares)	1,445,300		2,500,000		0
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased	3,000,000.0		3,000,000.0
Class B Common Stock
Class of Stock [Line Items]
Shares outstanding (in shares)	21,971,339		21,971,339		22,117,254
Director [Member] | Class A Common Stock
Class of Stock [Line Items]
Common Stock, Shares, Issued	19,491		19,491
Dividend, Share-based Payment Arrangement, Paid-in-Kind			$ 0.6