Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended			14 Months Ended	26 Months Ended
	Sep. 30, 2021	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2021	Dec. 31, 2020	Aug. 06, 2020	Jun. 10, 2014	Feb. 29, 2012
Class of Stock [Line Items]
Number of shares purchased from plan participants (in shares)	49,641
Dividend per common share (in usd per share)		$ 0	$ 0
Common stock
Class of Stock [Line Items]
Exercise of 3.25% Convertible Note (in shares)	3,031,817	3,050,850
3.25% Convertible Notes | Convertible Debt
Class of Stock [Line Items]
Stated interest rate									3.25%
Exercise of 3.25% Convertible Note (in shares)	3,031,817	3,050,850
2012 Repurchase Program
Class of Stock [Line Items]
Maximum number of shares authorized to be repurchased (in shares)										5,000,000
Shares repurchased under the program (in shares)			1,140,819	197,870	600,000
Aggregate cost of shares repurchased			$ 87.5	$ 16.0	$ 42.5		$ 204.6
2020 Repurchase Program
Class of Stock [Line Items]
Maximum number of shares authorized to be repurchased (in shares)								10,000,000,000,000
Shares repurchased under the program (in shares)		0				2,490,599
Aggregate cost of shares repurchased						$ 177.8