Stock Repurchase Program - Additional Information (Detail) $ / shares in Units, shares in Thousands	1 Months Ended					3 Months Ended				4 Months Ended	6 Months Ended
	Mar. 31, 2018 USD ($)	Feb. 28, 2018 USD ($)	Dec. 31, 2017 USD ($) share_repurchase_program	Nov. 30, 2017 USD ($)	Sep. 30, 2017 USD ($)	Apr. 30, 2018 USD ($) $ / shares shares	Jan. 31, 2018 USD ($)	Nov. 30, 2017 $ / shares shares	Apr. 30, 2017 shares	Mar. 31, 2018 $ / shares shares	Apr. 30, 2018 USD ($) shares	Apr. 30, 2017 shares	Apr. 05, 2018 USD ($)
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program authorized amount						$ 500,000,000					$ 500,000,000		$ 500,000,000
Remaining amount available for further repurchases						$ 490,000,000					$ 490,000,000
Shares repurchased (in shares) | shares						916			1,381		2,902	2,775
Accelerated Share Repurchase Program September 2017
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program authorized amount					$ 100,000,000.0
Prepayment amount					100,000,000.0
Accelerated share repurchase, initial share delivery, amount					$ 80,000,000.0
Stock repurchase program, prepayment during prior period, derivative settlement				$ 20,000,000.0			$ 20,000,000.0
Shares repurchased (in shares) | shares								1,200
Average purchase price of shares purchased under agreement (in USD per share) | $ / shares								$ 83.80
Accelerated Share Repurchase Program December 2017
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program authorized amount			$ 200,000,000.0
Prepayment amount			200,000,000.0
Accelerated share repurchase, initial share delivery, amount			$ 160,000,000.0
Stock repurchase program, prepayment during prior period, derivative settlement	$ 20,000,000.0	$ 20,000,000
Shares repurchased (in shares) | shares										2,300
Average purchase price of shares purchased under agreement (in USD per share) | $ / shares										$ 87.08
Accelerated share repurchase programs entered into | share_repurchase_program			2
Open Market Share Repurchase
Equity, Class of Treasury Stock [Line Items]
Shares repurchased (in shares) | shares						400
Average purchase price of shares purchased under agreement (in USD per share) | $ / shares						$ 82.61
Stock repurchased during period, value						$ 35,000,000