LONG-TERM INCENTIVE COMPENSATION (Details Textual) $ / shares in Units, CAD in Millions		3 Months Ended			9 Months Ended
	Apr. 26, 2017 shares	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2017 CAD shares	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Provision for income taxes		$ 1,620,000		$ 3,166,000	$ 7,013,000	$ 8,141,000
Amount of shares purchased in the open market					0	5,629,000
Remaining unrecognized compensation cost recognized over a remaining weighted average period		2 years 2 months 10 days	2 years 2 months 10 days
Accrued liability		$ 1,500,000			1,500,000		$ 1,700,000
Mutual Fund [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Mutual fund vesting period		1 year	1 year
Expense related to mutual fund share incentive awards		$ 281,000		313,000	$ 819,000	933,000
Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Shares Purchased for Award | shares	250,000
Total number of shares that may be issued under the stock based compensation Plan (including predecessor plans to the Plan) | shares		4,648,100			4,648,100
Shares remain available for issuance | shares		433,000			433,000
Remaining unrecognized compensation cost		$ 26,300,000			$ 26,300,000
Nonvested restricted shares | shares		532,297			532,297		607,501
Expense related to mutual fund share incentive awards		$ 4,233,000		$ 4,082,000	$ 12,298,000	$ 12,164,000
Canadian Plan | Westwood International Advisors Inc
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares remain available for issuance | shares		51,200			51,200
Share Based Compensation Arrangement By Share Based Payment Award Available For Grant, Original		$ 8,000,000	CAD 10.0
Fund purchases of common stock		55,418			$ 55,418
Purchases of common stock with respect to awards granted		$ 3,400,000	CAD 4.3
Share price | $ / shares		$ 67.27			$ 67.27
Amount of shares purchased in the open market		$ 1,300,000
Purchases of treasury stock, shares | shares		23,822	23,822
Remaining unrecognized compensation cost		$ 864,000			$ 864,000
Remaining unrecognized compensation cost recognized over a remaining weighted average period		1 year 10 months 3 days	1 year 10 months 3 days
Performance​ Based​ Restricted​ Shares​ For Future Performance Years
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Nonvested restricted shares | shares		16,313			16,313
Share-based Compensation Award, First 50% of CEO and CIO Award | Target [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share Pretax Income Goal					$ 24,000,000
Share-based Compensation Award, Second 50% of CEO and CIO Award | Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share Pretax Income Goal					$ 30,300,000
Percentage of Pretax income Goal Earned					25.00%
Share-based Compensation Award, Second 50% of CEO and CIO Award | Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share Pretax Income Goal					$ 42,500,000
Percentage of Pretax income Goal Earned					185.00%
Share-based Compensation Award, Second 50% of CEO and CIO Award | Target [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share Pretax Income Goal					$ 34,000,000
Share-based Compensation Award, All Other Performance Grants | Target [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share Pretax Income Goal					$ 24,000,000