Acquisitions QS Investors Acquisition (Details) £ in Thousands, $ in Thousands	3 Months Ended		6 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($)	Jun. 30, 2014 USD ($)	Sep. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2016 GBP (£)	Mar. 31, 2016 USD ($)	Sep. 30, 2017 GBP (£)	Sep. 30, 2017 USD ($)		Aug. 31, 2016 USD ($)	Oct. 01, 2014 USD ($)	May 30, 2014 USD ($)
Martin Currie [Member]
Business Acquisition [Line Items]
Business Combination, Contingent Consideration, Liability				$ 12,018		$ 41,222		$ 12,858			$ 75,211
Payments to Acquire Businesses, Gross					£ 125,000	202,577
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High							£ 325,000	435,884	[1]
QS Investors [Domain]
Business Acquisition [Line Items]
Business Combination, Contingent Consideration, Liability				4,841		$ 13,749		3,624				$ 13,370
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability	$ 1,217
Payments to Acquire Businesses, Gross		$ 11,000
Business Acquisition, Preacquisition Contingency, Amount of Settlement										$ 6,587
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High								23,400
Contingent payment due on second anniversary of acquisition | QS Investors [Domain]
Business Acquisition [Line Items]
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High								3,400
Contingent payment due on fourth anniversary of acquisition [Member] | QS Investors [Domain]
Business Acquisition [Line Items]
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High								$ 20,000
Other changes in Fair Value [Member] | Martin Currie [Member]
Business Acquisition [Line Items]
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability			$ (840)	4,204
Other changes in Fair Value [Member] | QS Investors [Domain]
Business Acquisition [Line Items]
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability			(83)	(179)
Change in Input Assumptions [Member] | Martin Currie [Member]
Business Acquisition [Line Items]
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability			0	25,000
Change in Input Assumptions [Member] | QS Investors [Domain]
Business Acquisition [Line Items]
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability	$ 1,300		$ 1,300	$ 2,500

[1] Using the applicable exchange rate as of September 30, 2017, for amounts denominated in currencies other than the U.S. dollar.