Fair Value Measurements and Long-term Investments - Summary of Long-term Equity Investments Included in Other Assets (Detail) $ in Thousands		9 Months Ended
		Sep. 30, 2021 USD ($) Investment	Dec. 31, 2020 USD ($)
Fair Value Disclosures [Abstract]
Equity method investments, Number of Investees | Investment	[1]	3
Cost less impairments, Number of investees | Investment		8
Total long-term equity investments, Number of Investees | Investment		11
Equity method investments, Original Cost	[1]	$ 7,099
Cost less impairment, Original Cost		49,336
Total long-term equity investments, Original Cost		56,435
Equity method investments, Carrying Value	[1]	10,181	$ 10,744
Cost less impairment, Carrying Value		49,127	25,059
Total long-term equity investments, Carrying Value		$ 59,308	$ 35,803

[1] Allscripts share of the earnings of our equity method investees is reported based on a one quarter lag.