Stockholders Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jul. 31, 2018
Stockholders Equity [Line Items]
Preferred stock, par value	$ 1
Common Stock, shares authorized	240,000,000
Common stock, par value	$ 1
Dividends paid	$ 0.76	$ 0.56	$ 0.48
Stock repurchase program number of shares authorized to repurchase	5,000,000			2,500,000
Shares repurchased	3,797,800	2,533,350	3,273,109
Shares repurchased, value	$ 202.6	$ 139.1	$ 135.2
Remaining authorization stock repurchase	6,075,253
Common Class A
Stockholders Equity [Line Items]
Common Stock, shares authorized	27,000,000
Common stock, par value	$ 5
Shares converted into Common Stock	48,232	73,792	60,045
Treasury stock, number of shares held	130,380		130,380
Common Stock
Stockholders Equity [Line Items]
Treasury stock, number of shares held	22,418,066		18,914,082