Fair Value of Financial Instruments - Financial Assets and Liabilities Carried at Fair Value (Detail) - USD ($) $ in Millions		Apr. 30, 2016	Apr. 30, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets, Fair Value		$ 55.4	$ 106.7
Total financial liabilities, Fair Value		17.3	10.0
Other assets, net [Member] | Marketable securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Assets, Fair Value	[1]	34.2	55.5
Short-term Investments [Member] | Bank Time Deposits [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Assets, Fair Value	[2]	10.3	39.6
Prepaid Expenses and Other Current Assets [Member] | Precious Metal Forward Contracts [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, Fair Value	[3]	9.2	0.3
Prepaid Expenses and Other Current Assets [Member] | Precious Metal Collars [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, Fair Value	[3]	0.9
Prepaid Expenses and Other Current Assets [Member] | Foreign Exchange Contract [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, Fair Value	[3]	0.6	10.3
Prepaid Expenses and Other Current Assets [Member] | Not Designated as Hedging Instrument [Member] | Foreign Exchange Contract [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, Fair Value	[3]	0.2	1.0
Accounts Payable and Accrued Liabilities [Member] | Precious Metal Forward Contracts [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value	[4]		4.7
Accounts Payable and Accrued Liabilities [Member] | Precious Metal Collars [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value	[4]	0.5
Accounts Payable and Accrued Liabilities [Member] | Foreign Exchange Contract [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value	[4]	14.8	0.3
Accounts Payable and Accrued Liabilities [Member] | Not Designated as Hedging Instrument [Member] | Foreign Exchange Contract [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value	[4]	2.0	5.0
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets, Fair Value		44.5	95.1
Total financial liabilities, Fair Value		0.0	0.0
Level 1 [Member] | Other assets, net [Member] | Marketable securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Assets, Fair Value	[1]	34.2	55.5
Level 1 [Member] | Short-term Investments [Member] | Bank Time Deposits [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Assets, Fair Value	[2]	10.3	39.6
Level 1 [Member] | Prepaid Expenses and Other Current Assets [Member] | Precious Metal Forward Contracts [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, Fair Value	[3]	0.0	0.0
Level 1 [Member] | Prepaid Expenses and Other Current Assets [Member] | Precious Metal Collars [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, Fair Value	[3]	0.0
Level 1 [Member] | Prepaid Expenses and Other Current Assets [Member] | Foreign Exchange Contract [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, Fair Value	[3]	0.0	0.0
Level 1 [Member] | Prepaid Expenses and Other Current Assets [Member] | Not Designated as Hedging Instrument [Member] | Foreign Exchange Contract [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, Fair Value	[3]	0.0	0.0
Level 1 [Member] | Accounts Payable and Accrued Liabilities [Member] | Precious Metal Forward Contracts [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value	[4]		0.0
Level 1 [Member] | Accounts Payable and Accrued Liabilities [Member] | Precious Metal Collars [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value	[4]	0.0
Level 1 [Member] | Accounts Payable and Accrued Liabilities [Member] | Foreign Exchange Contract [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value	[4]	0.0	0.0
Level 1 [Member] | Accounts Payable and Accrued Liabilities [Member] | Not Designated as Hedging Instrument [Member] | Foreign Exchange Contract [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value	[4]	0.0	0.0
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets, Fair Value		10.9	11.6
Total financial liabilities, Fair Value		17.3	10.0
Level 2 [Member] | Other assets, net [Member] | Marketable securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Assets, Fair Value	[1]	0.0	0.0
Level 2 [Member] | Short-term Investments [Member] | Bank Time Deposits [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Assets, Fair Value	[2]	0.0	0.0
Level 2 [Member] | Prepaid Expenses and Other Current Assets [Member] | Precious Metal Forward Contracts [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, Fair Value	[3]	9.2	0.3
Level 2 [Member] | Prepaid Expenses and Other Current Assets [Member] | Precious Metal Collars [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, Fair Value	[3]	0.9
Level 2 [Member] | Prepaid Expenses and Other Current Assets [Member] | Foreign Exchange Contract [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, Fair Value	[3]	0.6	10.3
Level 2 [Member] | Prepaid Expenses and Other Current Assets [Member] | Not Designated as Hedging Instrument [Member] | Foreign Exchange Contract [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, Fair Value	[3]	0.2	1.0
Level 2 [Member] | Accounts Payable and Accrued Liabilities [Member] | Precious Metal Forward Contracts [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value	[4]		4.7
Level 2 [Member] | Accounts Payable and Accrued Liabilities [Member] | Precious Metal Collars [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value	[4]	0.5
Level 2 [Member] | Accounts Payable and Accrued Liabilities [Member] | Foreign Exchange Contract [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value	[4]	14.8	0.3
Level 2 [Member] | Accounts Payable and Accrued Liabilities [Member] | Not Designated as Hedging Instrument [Member] | Foreign Exchange Contract [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value	[4]	2.0	5.0
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets, Fair Value		0.0	0.0
Total financial liabilities, Fair Value		0.0	0.0
Level 3 [Member] | Other assets, net [Member] | Marketable securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Assets, Fair Value	[1]	0.0	0.0
Level 3 [Member] | Short-term Investments [Member] | Bank Time Deposits [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Assets, Fair Value	[2]	0.0	0.0
Level 3 [Member] | Prepaid Expenses and Other Current Assets [Member] | Precious Metal Forward Contracts [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, Fair Value	[3]	0.0	0.0
Level 3 [Member] | Prepaid Expenses and Other Current Assets [Member] | Precious Metal Collars [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, Fair Value	[3]	0.0
Level 3 [Member] | Prepaid Expenses and Other Current Assets [Member] | Foreign Exchange Contract [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, Fair Value	[3]	0.0	0.0
Level 3 [Member] | Prepaid Expenses and Other Current Assets [Member] | Not Designated as Hedging Instrument [Member] | Foreign Exchange Contract [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, Fair Value	[3]	0.0	0.0
Level 3 [Member] | Accounts Payable and Accrued Liabilities [Member] | Precious Metal Forward Contracts [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value	[4]		0.0
Level 3 [Member] | Accounts Payable and Accrued Liabilities [Member] | Precious Metal Collars [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value	[4]	0.0
Level 3 [Member] | Accounts Payable and Accrued Liabilities [Member] | Foreign Exchange Contract [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value	[4]	0.0	0.0
Level 3 [Member] | Accounts Payable and Accrued Liabilities [Member] | Not Designated as Hedging Instrument [Member] | Foreign Exchange Contract [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value	[4]	$ 0.0	$ 0.0

[1] Included within Other assets, net.

[2] Included within Short-term investments.

[3] Included within Prepaid expenses and other current assets or Other assets, net evaluated based on the maturity of the contract.

[4] Included within Accounts payable and accrued liabilities or Other long-term liabilities evaluated based on the maturity of the contract.