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Stock-Based Compensation - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Feb. 28, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted (in shares)	0
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Vested in period, fair value	$ 32.5	$ 23.4
Unrecognized compensation cost	$ 28.2
Weighted-average remaining requisite service period	19 months
PAs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested in period, fair value	$ 1.6
Unrecognized compensation cost	$ 9.5
Weighted-average remaining requisite service period	12 months
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years		3 years
Weighted-average remaining requisite service period	11 months
Expiration period	10 years		10 years
Compensation not yet recognized	$ 1.1
Elanco Stock Compensation Plans
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock to be reserved for future issuance (in shares)	11,000,000		5,500,000	5,500,000
Stock-based compensation	$ 47.7	40.7	$ 1.8
Lilly's Stock-Based Compensation Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation	$ 0.0	$ 5.1	$ 26.0