Employee Stock and Benefit Plans (Options Outstanding and Vested) (Details) - USD ($) $ / shares in Units, $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2015
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Number of shares, outstanding			3,429,309
Weighted average exercise price per share, outstanding (in dollar per share)			$ 31.79
Weighted average remaining contractual life, outstanding			6 years 4 months 10 days
Number of shares, vested			2,459,936,000
Weighted average exercise price per share, vested (in dollar per share)			$ 25.27
Stock Options
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Weighted average grant date fair value (in dollars per share)	$ 33.05	$ 18.03		$ 11.12
Aggregate grant date fair value	$ 12.1	$ 7.4		$ 23.6
Intrinsic value of options exercised	$ 71.7	$ 92.1		$ 25.0
$1.90 - $1.90
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price range, lower limit (in dollar per share)	$ 2.60
Exercise price range, upper limit (in dollar per share)	4.40
Number of shares, outstanding			376,405
Weighted average exercise price per share, outstanding (in dollar per share)			$ 8.58
Weighted average remaining contractual life, outstanding			3 years 26 days
Number of shares, vested			376,405,000
Weighted average exercise price per share, vested (in dollar per share)			$ 8.58
$2.10 - $2.80
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price range, lower limit (in dollar per share)	4.80
Exercise price range, upper limit (in dollar per share)	13.60
Number of shares, outstanding			474,527
Weighted average exercise price per share, outstanding (in dollar per share)			$ 21.70
Weighted average remaining contractual life, outstanding			5 years 4 months 7 days
Number of shares, vested			412,956,000
Weighted average exercise price per share, vested (in dollar per share)			$ 21.18
$3.80 - $3.80
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price range, lower limit (in dollar per share)	16.68
Exercise price range, upper limit (in dollar per share)	25.17
Number of shares, outstanding			886,676
Weighted average exercise price per share, outstanding (in dollar per share)			$ 25.56
Weighted average remaining contractual life, outstanding			7 years 3 months
Number of shares, vested			587,978,000
Weighted average exercise price per share, vested (in dollar per share)			$ 25.56
$4.10 - $12.68
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price range, lower limit (in dollar per share)	25.56
Exercise price range, upper limit (in dollar per share)	25.56
Number of shares, outstanding			427,997
Weighted average exercise price per share, outstanding (in dollar per share)			$ 26.86
Weighted average remaining contractual life, outstanding			5 years 1 month
Number of shares, vested			427,997,000
Weighted average exercise price per share, vested (in dollar per share)			$ 26.86
$13.50 - $25.17
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price range, lower limit (in dollar per share)	26.58
Exercise price range, upper limit (in dollar per share)	26.86
Number of shares, outstanding			372,331
Weighted average exercise price per share, outstanding (in dollar per share)			$ 31.78
Weighted average remaining contractual life, outstanding			6 years 2 months 6 days
Number of shares, vested			307,743,000
Weighted average exercise price per share, vested (in dollar per share)			$ 31.54
$25.56 - $25.56
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price range, lower limit (in dollar per share)	30.58
Exercise price range, upper limit (in dollar per share)	31.67
Number of shares, outstanding			391,568
Weighted average exercise price per share, outstanding (in dollar per share)			$ 37.78
Weighted average remaining contractual life, outstanding			7 years 1 month 28 days
Number of shares, vested			225,200,000
Weighted average exercise price per share, vested (in dollar per share)			$ 37.53
$26.86 - $30.58
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price range, lower limit (in dollar per share)	34.97
Exercise price range, upper limit (in dollar per share)	37.28
Number of shares, outstanding			213,744
Weighted average exercise price per share, outstanding (in dollar per share)			$ 49.97
Weighted average remaining contractual life, outstanding			7 years 6 months 20 days
Number of shares, vested			121,460,000
Weighted average exercise price per share, vested (in dollar per share)			$ 45.16
$31.67 - $37.28
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price range, lower limit (in dollar per share)	38.40
Exercise price range, upper limit (in dollar per share)	40.89
Number of shares, outstanding			101,125
Weighted average exercise price per share, outstanding (in dollar per share)			$ 76.21
Weighted average remaining contractual life, outstanding			9 years 9 months 10 days
Number of shares, vested			0
Weighted average exercise price per share, vested (in dollar per share)			$ 0.00
$40.68 - $40.89
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price range, lower limit (in dollar per share)	52.14
Exercise price range, upper limit (in dollar per share)	52.14
Number of shares, outstanding			78,550
Weighted average exercise price per share, outstanding (in dollar per share)			$ 78.85
Weighted average remaining contractual life, outstanding			9 years 3 months 19 days
Number of shares, vested			93,000
Weighted average exercise price per share, vested (in dollar per share)			$ 78.85
$52.14 - $52.14
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price range, lower limit (in dollar per share)	52.60
Exercise price range, upper limit (in dollar per share)	$ 52.60
Number of shares, outstanding			106,386
Weighted average exercise price per share, outstanding (in dollar per share)			$ 95.10
Weighted average remaining contractual life, outstanding			9 years 5 months 26 days
Number of shares, vested			104,000
Weighted average exercise price per share, vested (in dollar per share)			$ 95.10