Statement of Cash Flows - Narrative (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Supplemental Cash Flow Information [Abstract]
Accrued capital expenditures	$ 72.6	$ 133.7	$ 54.4	$ 54.5

Carrying value as of the balance sheet date of obligations incurred and payable for purchases of fixed assets. Included in accounts payable and accrued expenses.

No definition available.

Name:	regn_AccruedCapitalExpenditures
Namespace Prefix:	regn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant