Acquisitions Martin Currie Acquisition (Details) £ in Thousands, AUD in Thousands, $ in Thousands	3 Months Ended				6 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 AUD	Sep. 30, 2016 USD ($)	Jun. 30, 2014 USD ($)	Sep. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2016 GBP (£)	Mar. 31, 2016 USD ($)	Sep. 30, 2017 AUD	Sep. 30, 2017 GBP (£)	Sep. 30, 2017 USD ($)		Oct. 21, 2015 USD ($)	Oct. 01, 2014 USD ($)	May 30, 2014 USD ($)
QS Investors [Domain]
Business Acquisition [Line Items]
Business Combination, Contingent Consideration, Liability, Current											$ 3,624
Business Combination, Contingent Consideration, Liability						$ 4,841		$ 13,749			3,624				$ 13,370
Payments to Acquire Businesses, Gross				$ 11,000
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High											23,400
Contingent consideration fair value adjustments	$ (1,217)
RARE Infrastructure, Ltd [Member]
Business Acquisition [Line Items]
Business Combination, Contingent Consideration, Liability, Current						7,791					0
Assets Under Management													$ 6,800,000
Business Combination, Contingent Consideration, Liability						17,444		27,145			2,180		$ 25,000
Payments to Acquire Businesses, Gross		AUD 296,000	$ 213,739
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High									AUD 106,000		83,174	[1]
Contingent consideration fair value adjustments					$ (15,264)
Martin Currie [Member]
Business Acquisition [Line Items]
Business Combination, Contingent Consideration, Liability, Current											12,858
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]
Change in Input Assumptions [Member] | QS Investors [Domain]
Business Acquisition [Line Items]
Contingent consideration fair value adjustments	$ (1,300)				(1,300)	(2,500)
Change in Input Assumptions [Member] | RARE Infrastructure, Ltd [Member]
Business Acquisition [Line Items]
Contingent consideration fair value adjustments					(15,250)	(10,000)
Change in Input Assumptions [Member] | Martin Currie [Member]
Business Acquisition [Line Items]
Contingent consideration fair value adjustments					0	(25,000)
Other changes in Fair Value [Member] | QS Investors [Domain]
Business Acquisition [Line Items]
Contingent consideration fair value adjustments					83	179
Other changes in Fair Value [Member] | RARE Infrastructure, Ltd [Member]
Business Acquisition [Line Items]
Contingent consideration fair value adjustments					(14)	299
Other changes in Fair Value [Member] | Martin Currie [Member]
Business Acquisition [Line Items]
Contingent consideration fair value adjustments					$ 840	$ (4,204)

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