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Segregated Funds - Summary of Composition of Net Assets by Categories of Segregated Funds (Detail)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Bottom of range [Member] | Money market funds [Member]    
Disclosure of composition of net assets by categories of segregated funds [line items]    
Type of fund 2.00% 2.00%
Bottom of range [Member] | Fixed income funds [Member]    
Disclosure of composition of net assets by categories of segregated funds [line items]    
Type of fund 14.00% 14.00%
Bottom of range [Member] | Balanced funds [Member]    
Disclosure of composition of net assets by categories of segregated funds [line items]    
Type of fund 22.00% 22.00%
Bottom of range [Member] | Equity funds [Member]    
Disclosure of composition of net assets by categories of segregated funds [line items]    
Type of fund 55.00% 59.00%
Top of range [Member] | Money market funds [Member]    
Disclosure of composition of net assets by categories of segregated funds [line items]    
Type of fund 3.00% 3.00%
Top of range [Member] | Fixed income funds [Member]    
Disclosure of composition of net assets by categories of segregated funds [line items]    
Type of fund 15.00% 15.00%
Top of range [Member] | Balanced funds [Member]    
Disclosure of composition of net assets by categories of segregated funds [line items]    
Type of fund 29.00% 24.00%
Top of range [Member] | Equity funds [Member]    
Disclosure of composition of net assets by categories of segregated funds [line items]    
Type of fund 60.00% 61.00%