Schedule I - Condensed Financial Information of Registrant - Condensed Statements of Financial Condition, Additional Information (Details) - $ / shares	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Balance Sheet Statements, Captions [Line Items]
Common shares, par value (USD per share)	$ 1	$ 1	$ 1
Common shares, authorized	600,000,000	600,000,000
Common shares, issued and outstanding after deducting shares held in treasury	356,227,038	359,425,061
Treasury stock, shares	60,165,980	56,947,654
Parent Company
Condensed Balance Sheet Statements, Captions [Line Items]
Common shares, par value (USD per share)	$ 1	$ 1
Common shares, authorized	600,000,000	600,000,000
Common shares, issued and outstanding after deducting shares held in treasury	356,227,038	359,425,061
Treasury stock, shares	60,165,980	56,947,654