Stock-based Compensation Expense and Share Repurchase - Stock Options Outstanding and Exercisable (Details) shares in Thousands	9 Months Ended
	Sep. 30, 2018 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Exercise price range, options outstanding (in shares) | shares	8,945
Exercise price range, options outstanding, weighted-average remaining contractual life (in years)	7 years 2 months 5 days
Exercise price range, options outstanding, weighted-average exercise price (in dollars per share)	$ 111.22
Exercise price range, options exercisable (in shares) | shares	4,313
Exercise price range, options exercisable, weighted-average exercise price (in dollars per share)	$ 89.83
$29.07–$40.00
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Exercise price, low end of range (in dollars per share)	29.07
Exercise price, high end of range (in dollars per share)	$ 40
Exercise price range, options outstanding (in shares) | shares	550
Exercise price range, options outstanding, weighted-average remaining contractual life (in years)	1 year 4 months 10 days
Exercise price range, options outstanding, weighted-average exercise price (in dollars per share)	$ 34.89
Exercise price range, options exercisable (in shares) | shares	550
Exercise price range, options exercisable, weighted-average exercise price (in dollars per share)	$ 34.89
$40.01–$60.00
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Exercise price, low end of range (in dollars per share)	40.01
Exercise price, high end of range (in dollars per share)	$ 60
Exercise price range, options outstanding (in shares) | shares	495
Exercise price range, options outstanding, weighted-average remaining contractual life (in years)	3 years 8 months 16 days
Exercise price range, options outstanding, weighted-average exercise price (in dollars per share)	$ 49.82
Exercise price range, options exercisable (in shares) | shares	495
Exercise price range, options exercisable, weighted-average exercise price (in dollars per share)	$ 49.82
$60.01–$80.00
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Exercise price, low end of range (in dollars per share)	60.01
Exercise price, high end of range (in dollars per share)	$ 80
Exercise price range, options outstanding (in shares) | shares	556
Exercise price range, options outstanding, weighted-average remaining contractual life (in years)	5 years 6 months 4 days
Exercise price range, options outstanding, weighted-average exercise price (in dollars per share)	$ 74.96
Exercise price range, options exercisable (in shares) | shares	545
Exercise price range, options exercisable, weighted-average exercise price (in dollars per share)	$ 74.94
$80.01–$100.00
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Exercise price, low end of range (in dollars per share)	80.01
Exercise price, high end of range (in dollars per share)	$ 100
Exercise price range, options outstanding (in shares) | shares	3,017
Exercise price range, options outstanding, weighted-average remaining contractual life (in years)	7 years 5 months 16 days
Exercise price range, options outstanding, weighted-average exercise price (in dollars per share)	$ 89.18
Exercise price range, options exercisable (in shares) | shares	1,228
Exercise price range, options exercisable, weighted-average exercise price (in dollars per share)	$ 89.88
$100.01–$120.00
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Exercise price, low end of range (in dollars per share)	100.01
Exercise price, high end of range (in dollars per share)	$ 120
Exercise price range, options outstanding (in shares) | shares	725
Exercise price range, options outstanding, weighted-average remaining contractual life (in years)	6 years 4 months 10 days
Exercise price range, options outstanding, weighted-average exercise price (in dollars per share)	$ 109.30
Exercise price range, options exercisable (in shares) | shares	550
Exercise price range, options exercisable, weighted-average exercise price (in dollars per share)	$ 109.19
$120.01–$140.00
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Exercise price, low end of range (in dollars per share)	120.01
Exercise price, high end of range (in dollars per share)	$ 140.00
Exercise price range, options outstanding (in shares) | shares	880
Exercise price range, options outstanding, weighted-average remaining contractual life (in years)	6 years 10 months 21 days
Exercise price range, options outstanding, weighted-average exercise price (in dollars per share)	$ 130.25
Exercise price range, options exercisable (in shares) | shares	615
Exercise price range, options exercisable, weighted-average exercise price (in dollars per share)	$ 130.15
$140.01–$160.00
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Exercise price, low end of range (in dollars per share)	140.01
Exercise price, high end of range (in dollars per share)	$ 160.00
Exercise price range, options outstanding (in shares) | shares	1,445
Exercise price range, options outstanding, weighted-average remaining contractual life (in years)	9 years 3 months 29 days
Exercise price range, options outstanding, weighted-average exercise price (in dollars per share)	$ 155.52
Exercise price range, options exercisable (in shares) | shares	198
Exercise price range, options exercisable, weighted-average exercise price (in dollars per share)	$ 155.20
$160.01–$180.00
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Exercise price, low end of range (in dollars per share)	160.01
Exercise price, high end of range (in dollars per share)	$ 180.00
Exercise price range, options outstanding (in shares) | shares	545
Exercise price range, options outstanding, weighted-average remaining contractual life (in years)	8 years 9 months 7 days
Exercise price range, options outstanding, weighted-average exercise price (in dollars per share)	$ 162.94
Exercise price range, options exercisable (in shares) | shares	128
Exercise price range, options exercisable, weighted-average exercise price (in dollars per share)	$ 162.94
$180.01–$181.60
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Exercise price, low end of range (in dollars per share)	180.01
Exercise price, high end of range (in dollars per share)	$ 181.60
Exercise price range, options outstanding (in shares) | shares	732
Exercise price range, options outstanding, weighted-average remaining contractual life (in years)	9 years 9 months 18 days
Exercise price range, options outstanding, weighted-average exercise price (in dollars per share)	$ 181.60
Exercise price range, options exercisable (in shares) | shares	4
Exercise price range, options exercisable, weighted-average exercise price (in dollars per share)	$ 181.60