Commitments and Contingencies Commitments and Contingencies related to Acquisitions (Details) £ in Thousands, AUD in Thousands, $ in Thousands		3 Months Ended	12 Months Ended
		Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)		Mar. 31, 2016 USD ($)		Mar. 31, 2015 USD ($)	Mar. 31, 2017 AUD	Mar. 31, 2017 GBP (£)	Aug. 31, 2016 USD ($)	Oct. 21, 2015 USD ($)	May 05, 2015 USD ($)	Oct. 01, 2014 USD ($)	May 30, 2014 USD ($)	Mar. 31, 2014 USD ($)
Business Acquisition, Contingent Consideration [Line Items]
Redeemable Noncontrolling Interest, Equity, Carrying Amount			$ 677,772		$ 175,785		$ 45,520								$ 45,144
RARE Infrastructure, Ltd [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration, Liability			17,444		27,145		0				$ 25,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Contingent Liability	[1]										$ 25,000
Business Combination, Contingent Consideration, Liability, Current			7,791
Business Combination, Contingent Consideration, Liability, Noncurrent			9,653
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability			(9,701)
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High			81,063	[2]				AUD 106,000
Martin Currie [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration, Liability			12,018		41,222		70,114						$ 75,211
Business Combination, Contingent Consideration, Liability, Current			12,018
Business Combination, Contingent Consideration, Liability, Noncurrent			0
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability			(29,204)
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High			407,403	[2]					£ 325,000
PK Investments [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High	[2]		2,507
QS Investors [Domain]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration, Liability			4,841		13,749		13,553							$ 13,370
Business Combination, Contingent Consideration, Liability, Current			0
Business Combination, Contingent Consideration, Liability, Noncurrent			4,841
Business Acquisition, Preacquisition Contingency, Amount of Settlement										$ (6,587)
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability			(8,908)
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High			23,400
Fauchier [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration, Liability			0		0		27,117
Business Combination, Contingent Consideration, Liability, Current			0
Business Combination, Contingent Consideration, Liability, Noncurrent			0
Business Acquisition, Preacquisition Contingency, Amount of Settlement										$ (6,587)		$ (22,765)
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High	[2]		0
Other Affiliates [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration, Liability	[3]		2,507		2,469		0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Contingent Liability	[3]		2,000		2,457	[1]
Business Combination, Contingent Consideration, Liability, Current	[3]		2,507
Business Combination, Contingent Consideration, Liability, Noncurrent	[3]		0
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High	[3]		5,507
Other changes in Fair Value [Member] | RARE Infrastructure, Ltd [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability			299		2,145
Other changes in Fair Value [Member] | Martin Currie [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability			(4,204)		(531)
Other changes in Fair Value [Member] | QS Investors [Domain]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability			179		196
Other changes in Fair Value [Member] | Fauchier [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability			0		662
Other changes in Fair Value [Member] | Other Affiliates [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability	[3]		38		12
Change in Input Assumptions [Member] | RARE Infrastructure, Ltd [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability			(10,000)
Change in Input Assumptions [Member] | Martin Currie [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability			(25,000)		(28,361)
Change in Input Assumptions [Member] | QS Investors [Domain]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability			(2,500)
Change in Input Assumptions [Member] | Fauchier [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability					(5,014)
Change in Input Assumptions [Member] | Other Affiliates [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability	[3]		(2,000)
Contingent payment due on fourth anniversary of acquisition [Member] | QS Investors [Domain]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High			20,000
Management Equity Plan [Member]
Business Acquisition, Contingent Consideration [Line Items]
Deferred Compensation Arrangement with Individual, Fair Value of Shares Issued			72,298
Consolidated Legg Mason, Inc.
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration, Liability			36,810		84,585		110,784
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Contingent Liability			2,000		27,457
Business Combination, Contingent Consideration, Liability, Current			22,316		26,396
Business Combination, Contingent Consideration, Liability, Noncurrent			14,494		58,189
Business Acquisition, Preacquisition Contingency, Amount of Settlement			6,587		22,765		0
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High	[2]		517,373
Redeemable Noncontrolling Interest, Equity, Carrying Amount			677,772		175,785
Consolidated Legg Mason, Inc. | Other changes in Fair Value [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability			(3,688)		2,484
Consolidated Legg Mason, Inc. | Change in Input Assumptions [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability			(39,500)		(33,375)		0
Subsequent Event [Member] | Change in Input Assumptions [Member] | RARE Infrastructure, Ltd [Member]
Business Acquisition, Contingent Consideration [Line Items]
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability		$ 11,500
Noncontrolling Interest [Member]
Business Acquisition, Contingent Consideration [Line Items]
Redeemable Noncontrolling Interest, Equity, Carrying Amount			$ 591,254		$ 68,922		$ 1,949								$ 1,356

[1] Using the applicable exchange rate on the date of acquisition for amounts denominated in currencies other than the U.S. dollar.

[2] Using the applicable exchange rate as of March 31, 2017, for amounts denominated in currencies other than the U.S. dollar.

[3] Includes amounts related to the acquisition of Financial Guard on August 17, 2016 and PK Investments on December 31, 2015.