Stock-Based Compensation - Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Stock-based compensation
Stock-based compensation expense	$ 10.0	$ 7.8	$ 6.9
Related income tax benefit of stock-based compensation	$ 2.4	$ 2.0	$ 1.8
Employee stock-based compensation plan
Stock-based compensation
Total shares authorized	5,600,000
Shares available for future grants	3,800,000
Non-employee director plan
Stock-based compensation
Total shares authorized	75,000
Shares available for future grants	6,059