Share-Based Compensation - RSU and RSA activity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
RSUs
RSU and RSA activity ( including Performance RSUs and RSAs)
Outstanding at beginning of year	4,939,774
Granted (in shares)	2,576,326
Released (in shares)	(1,907,307)
Forfeited (in shares)	(614,875)
Outstanding at end of year	4,993,918	4,939,774
Additional disclosures
Aggregate intrinsic value outstanding	$ 88.7
Total estimated fair value of RSUs vested	$ 46.8	$ 32.4
Performance-Contingent Awards - RSAs
RSU and RSA activity ( including Performance RSUs and RSAs)
Outstanding at beginning of year	776,250
Granted (in shares)	0
Released (in shares)	(362,250)
Forfeited (in shares)	0
Outstanding at end of year	414,000	776,250
Additional disclosures
Aggregate intrinsic value outstanding	$ 7.4