Non-Controlling Interests, Preferred Shares, Common Shares and Common Shares in Treasury - Common Share Dividends Declared (Detail) - $ / shares	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity [Abstract]
Common share dividends declared per share	$ 0.76	$ 0.76	$ 0.69

Aggregate dividends declared during the period for each share of common stock outstanding.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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_CommonStockDividendsPerShareDeclared
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration