Equity impact of the cost of common and preferred stock that were repurchased during the period. Recorded using the cost method.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6405813&loc=d3e23239-112655

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Name:	us-gaap_TreasuryStockValueAcquiredCostMethod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Stock Repurchases - Additional Information (Detail) - Class A Common Shares - USD ($) $ in Millions	9 Months Ended	197 Months Ended
	Jan. 31, 2019	Jan. 31, 2019	Aug. 19, 2002
Equity, Class of Treasury Stock [Line Items]
Common stock shares repurchased	4,588,632
Cost of common stock repurchased	$ 25.6
Shares Stock Repurchase Plan, August 19, 2002
Equity, Class of Treasury Stock [Line Items]
Approved number of shares to be repurchased			2,000,000
Common stock shares repurchased		1,053,679
Cost of common stock repurchased		$ 6.2