Stock Plans (Restricted Shares Outstanding) (Details) - Restricted Stock [Member] - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Balance at beginning of period (in shares)	900	1,858	633
Restricted stock granted (in shares)		105	2,023
Restricted stock vested (in shares)	(387)	(1,039)	(221)
Restricted stock forfeited (in shares)	(209)	(24)	(577)
Balance at end of period (in shares)	304	900	1,858
Balance at beginning of period (in dollars per shares)	$ 10.57	$ 16.02	$ 58.63
Restricted stock granted (in dollars per shares)	0.00	2.00	8.26
Restricted stock vested (in dollars per shares)	15.04	19.05	66.82
Restricted stock forfeited (in dollars per shares)	7.79	28.30	16.47
Balance at end of period (in dollars per shares)	$ 6.78	$ 10.57	$ 16.02
Balance at beginning of period	$ 1	$ 4	$ 4
Restricted stock granted			5
Restricted stock vested		(1)	(1)
Balance at end of period		$ 1	$ 4