Stockholders' Equity, Restricted Stock Unit Activity (Details) - Restricted Stock Units [Member] - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Number of Shares [Abstract]
Non-vested at beginning of period (in shares)	1,866
Granted (in shares)	1,244
Vested (in shares)	(602)
Cancelled/forfeited (in shares)	(134)
Non-vested at end of period (in shares)	2,374	1,866
Weighted Average Grant Date Fair Value per Share [Abstract]
Non-vested at beginning of period (in dollars per share)	$ 55.80
Granted (in dollars per share)	60.86	$ 60.23	$ 51.06
Vested (in dollars per share)	53.68
Cancelled/forfeited (in dollars per share)	58.96
Non-vested at end of period (in dollars per share)	$ 58.81	$ 55.80
Unrecognized Compensation Expense [Abstract]
Unrecognized compensation cost related to non-vested RSUs	$ 55.3
Weighted average period for recognition	1 year 3 months 18 days