Goodwill and Acquired Intangible Assets (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Goodwill And Intangible Assets Disclosure [Abstract]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	9 years
Amortization expense	$ 2.0	$ 2.0	$ 2.3
Finite-Lived Intangible Assets, Estimated Amortization Expense, 2020	1.9
Finite-Lived Intangible Assets, Estimated Amortization Expense, 2021	1.9
Finite-Lived Intangible Assets, Estimated Amortization Expense, 2022	1.9
Finite-Lived Intangible Assets, Estimated Amortization Expense, 2023	1.4
Finite-Lived Intangible Assets, Estimated Amortization Expense, 2024	$ 0.3