Cash Equivalents and Investments - Tabular Disclosure - Total (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Cash Equivalents and Investments
Total, Amortized Cost	$ 743,997	$ 547,454
Total, Unrealized Gain	2,975	518
Total, Unrealized Losses		(12)
Total, Fair Value	$ 746,972	$ 547,960

Represents the cost of cash equivalents and cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

No definition available.

Name:	bpmc_CashEquivalentAndAvailableForSaleDebtSecuritiesAmortizedCostBasis
Namespace Prefix:	bpmc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant