Stock Plans (Restricted Shares Outstanding) (Details) - Restricted Stock [Member] - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Balance at beginning of period (in shares)	549	462	512
Shares granted (in shares)	454	396	188
Shares vested (in shares)	(240)	(248)	(214)
Shares forfeited (in shares)	(130)	(61)	(24)
Balance at end of period (in shares)	633	549	462
Balance at beginning of period (in dollars per shares)	$ 78.00	$ 88.95	$ 71.25
Restricted stock granted (in dollars per shares)	47.77	65.40	118.80
Restricted stock vested (in dollars per shares)	80.86	78.90	73.35
Restricted stock forfeited (in dollars per shares)	60.92	76.65	76.50
Balance at end of period (in dollars per shares)	$ 58.63	$ 78.00	$ 88.95
Balance at beginning of period	$ 28	$ 33	$ 52
Restricted stock granted	3	20	13
Restricted stock vested	2	13	15
Balance at end of period	$ 4	$ 28	$ 33