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Stock-Based Compensation - Restricted Stock Awards (Details) - Restricted Stock Awards - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Number of Shares
Unvested, beginning balance (in shares)			5
Granted (in shares)			4
Vested (in shares)			(3)
Forfeited (in shares)			0
Unvested, ending balance (in shares)	6		6
Weighted Average Grant Date Fair Value per Share
Unvested, beginning balance (in dollars per share)			$ 153.41
Granted (in dollars per share)			144.33
Vested (in dollars per share)			160.34
Forfeited (in dollars per share)			0
Unvested, ending balance (in dollars per share)	$ 143.74		$ 143.74
Vesting period			1 year
Stock-based compensation expense	$ 0.2	$ 0.2	$ 0.3	$ 0.3
Remaining stock-based compensation expense for unvested shares, not yet recognized	$ 0.7		$ 0.7
Stock-based compensation expense, weighted average recognition period			10 months 24 days