Goodwill and Acquired Intangible Assets (Narrative) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2021 USD ($)
Goodwill And Intangible Assets Disclosure [Abstract]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	9 years
Finite-Lived Intangible Assets, Estimated Amortization Expense, 2021	$ 0.6
Finite-Lived Intangible Assets, Estimated Amortization Expense, December 31, 2022	2.2
Finite-Lived Intangible Assets, Estimated Amortization Expense, December 31, 2023	1.7
Finite-Lived Intangible Assets, Estimated Amortization Expense, December 31, 2024	0.6
Finite-Lived Intangible Assets, Estimated Amortization Expense, December 31, 2025	$ 0.6