Stock Plans (Restricted Shares Outstanding) (Details) - Restricted Stock [Member] - USD ($) $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Balance at beginning of period (in shares)	549,000
Shares granted (in shares)	454,000	375,133
Shares earned (in shares)	(220,000)
Shares forfeited (in shares)	(119,000)
Balance at end of period (in shares)	664,000
Balance at beginning of period (in dollars per shares)	$ 78.00
Restricted stock granted (in dollars per shares)	47.77
Restricted stock vested (in dollars per shares)	79.78
Restricted stock forfeited (in dollars per shares)	61.21
Balance at end of period (in dollars per shares)	$ 59.64	$ 78.60
Balance at beginning of period, Aggregate fair value	$ 28
Restricted stock granted, Aggregate fair value	5	$ 23
Restricted stock vested, Aggregate fair value	(3)	(15)
Balance at end of period, Aggregate fair value	$ 8	$ 37