Earnings per Share - Schedule of Earnings Per Share - Basic and Diluted (Details) - Common Class A - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Net income attributable to HLI
Basic EPS of Class A common stock	$ 15,299	$ 11,222		$ 20,067
Assumed vesting of employee awards	110	165	$ 181	282
Diluted EPS of Class A common stock	$ 15,409	$ 11,387	$ 26,861	$ 20,349
Weighted-Average Shares
Weighted-average basic EPS of Class A common stock (in shares)	27,229,152	22,671,865	26,969,363	22,461,363
Weighted-average assumed vesting of employee awards (in shares)	403,738	585,554	372,231	554,082
Weighted-average diluted EPS of Class A common stock (in shares)	27,632,890	23,257,419	27,341,594	23,015,445
Per share amount
Basic EPS of Class A common stock (in dollars per share)	$ 0.56	$ 0.49	$ 0.99	$ 0.89
Diluted EPS of Class A common stock (in dollars per share)	$ 0.56	$ 0.49	$ 0.98	$ 0.88