Stock-based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Weighted Average Exercise Price
Forfeited weighted average exercise price (dollars per share)	$ 0	$ 0
Stock Option Plans 2003 and 2013
Shares
Beginning balance (in shares)	5.2	6.4
Granted (in shares)	0.0	0.0
Exercised (in shares)	(0.5)	(1.2)
Forfeited (in shares)	0.0	0.0
Ending balance (in shares)	4.7	5.2
Options exercisable (in shares)	3.6
Weighted Average Exercise Price
Beginning balance weighted average exercise price (dollars per share)	$ 16.30	$ 15.63
Granted weighted average exercise price (dollars per share)	0	0
Exercised weighted average exercise price (in dollars per share)	13.03	13.12
Ending balance weighted average exercise price (dollars per share)	16.69	$ 16.30
Options exercisable weighted average exercise price (dollars per share)	$ 16.78
Options outstanding weighted average remaining contractual term (in years)	5 years 7 months 2 days
Options outstanding weighted average intrinsic value	$ 47.7
Options exercisable weighted average remaining contractual term (in years)	5 years 3 months 25 days
Options exercisable weighted average intrinsic value	$ 36.3

The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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- Definition

The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.