Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019
Class of Stock [Line Items]
Total revenues	$ 102,877	$ 84,583	$ 84,431	$ 69,744	$ 76,932	$ 68,138	$ 64,292	$ 64,686	$ 341,635	$ 274,048	$ 252,179
Total expenses	57,015	41,864	46,117	40,911	43,313	39,246	37,367	37,693	185,907	157,619	147,955
Net income	64,586	54,632	44,599	4,968	38,613	28,437	32,273	27,453	168,785	126,776	98,997
Net income attributable to HLI	$ 39,760	$ 33,203	$ 21,813	$ 3,246	$ 20,648	$ 13,497	$ 15,299	$ 11,381	$ 98,022	$ 60,825	$ 33,573
Earnings per share of Class A common stock:
Diluted earnings per share of Class A common stock (in dollars per share)									$ 2.81	$ 2.15	$ 1.40
Common Class A
Class of Stock [Line Items]
Net income attributable to HLI									$ 98,022	$ 60,825	$ 33,573
Earnings per share of Class A common stock:
Basic earnings per share of Class A common stock (in dollars per share)	$ 1.01	$ 0.96	$ 0.66	$ 0.11	$ 0.71	$ 0.46	$ 0.56	$ 0.43	$ 2.82	$ 2.17	$ 1.41
Diluted earnings per share of Class A common stock (in dollars per share)	$ 1.00	$ 0.96	$ 0.66	$ 0.11	$ 0.70	$ 0.46	$ 0.56	$ 0.42	$ 2.81	$ 2.15	$ 1.40