Equity Arrangements - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended		3 Months Ended
	Mar. 31, 2018	Mar. 31, 2015	Mar. 31, 2018	Jul. 01, 2017	Apr. 01, 2017	Dec. 30, 2017	May 17, 2017	May 11, 2017
Stockholders Equity Note [Line Items]
Interest Expense			$ 1.3
Forward share purchase contract		$ 350.0
Payments for Repurchase of Common Stock			$ 11.4		$ 13.5
Common Stock, Shares, Issued	176,902,738		176,902,738			176,902,738
Long-term Debt			$ 750.0
Number of common shares purchased under call option			3,200,000	4,600,000
Call option, aggregate premium			$ 57.3	$ 25.1
Preferred Stock, Value, Issued	$ 750.0		$ 750.0			$ 750.0
Equity Forward Contracts, Net Settlement, Shares		3,645,510
Equity Units Conversion Rate Number Of Common Stock Shares			0.7248	0.7241
Document Period End Date			Mar. 31, 2018
equity units issued				7,500,000
Equity Unit				$ 750.0
Shares Issued, Price Per Share	$ 137.97		$ 137.97	$ 100				$ 138.10
Forward Contract Indexed to Issuer's Equity, Forward Rate Per Share			100	100
Preferred Stock, Liquidation Preference Per Share				$ 1,000
equity unit proceeds				$ 727.5
Preferred Stock, Shares Issued				750,000
Preferred Stock, Liquidation Preference, Value				$ 750.0
Preferred Stock Conversion Rate Number Of Common Stock Shares			6.1687	$ 6.1627
Option Indexed to Issuer's Equity, Strike Price			$ 162.11	162.27
Forward Contract Indexed to Issuer's Equity, Shares			5,400,000
Option Indexed to Issuer's Equity, Settlement Alternatives, Cash, at Fair Value	$ 53.6		$ 53.6
Percentage Of Contract Price Paid	5.375%		5.375%
Forward Contract Indexed to Issuer's Equity, Settlement Alternatives, Cash, at Fair Value	$ 88.0		$ 88.0				$ 117.1
Call Option [Member]
Stockholders Equity Note [Line Items]
Call option, average price (in dollars per share)			$ 17.96	5.43
Maximum [Member]
Stockholders Equity Note [Line Items]
Option Indexed to Issuer's Equity, Strike Price	$ 203.92		179.35	179.53
Minimum [Member]
Stockholders Equity Note [Line Items]
Option Indexed to Issuer's Equity, Strike Price	$ 156.86		$ 162.11	$ 162.27
May 2017 Capped Call [Member]
Stockholders Equity Note [Line Items]
Option Indexed to Issuer's Equity, Settlement Alternatives, Cash, at Fair Value	$ 31.0		$ 31.0