Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule Of Equity [Line Items]
Cash Dividend on Common Stock	$ 0.80	$ 0.80	$ 0.80	$ 0.75	$ 2.40	$ 2.25
Aggregated Dividend Paid	$ 41.0	$ 41.0	$ 40.9
Dividend payable date declared	Sep. 18, 2017	Jun. 09, 2017	Feb. 28, 2017
Common stock Dividend Payable Date	Oct. 13, 2017	Jul. 14, 2017	Apr. 14, 2017
Dividend payable date of record	Sep. 29, 2017	Jun. 19, 2017	Mar. 31, 2017
2015 Stock Repurchase Program [Member]
Schedule Of Equity [Line Items]
Company repurchased, shares						0.5
Repurchase of Company stock for retirement						$ 24.8