Stock-based Compensation Expense and Share Repurchase - Stock Options Outstanding and Exercisable (Details) shares in Thousands	6 Months Ended
	Jun. 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	9,312
Exercise price range, options outstanding, weighted-average remaining contractual life (in years)	7 years 1 month 2 days
Exercise price range, options outstanding, weighted-average exercise price (in dollars per share)	$ 103.58
Exercise price range, options exercisable (in shares) | shares	4,764
Exercise price range, options exercisable, weighted-average exercise price (in dollars per share)	$ 87.14
$26.73–$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)	26.73
Exercise price, high end of range (in dollars per share)	$ 40
Exercise price range, options outstanding (in shares) | shares	707
Exercise price range, options outstanding, weighted-average remaining contractual life (in years)	1 year 4 months 13 days
Exercise price range, options outstanding, weighted-average exercise price (in dollars per share)	$ 34.48
Exercise price range, options exercisable (in shares) | shares	707
Exercise price range, options exercisable, weighted-average exercise price (in dollars per share)	$ 34.48
$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	557
Exercise price range, options outstanding, weighted-average remaining contractual life (in years)	4 years 4 days
Exercise price range, options outstanding, weighted-average exercise price (in dollars per share)	$ 49.57
Exercise price range, options exercisable (in shares) | shares	557
Exercise price range, options exercisable, weighted-average exercise price (in dollars per share)	$ 49.57
$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	615
Exercise price range, options outstanding, weighted-average remaining contractual life (in years)	5 years 9 months
Exercise price range, options outstanding, weighted-average exercise price (in dollars per share)	$ 75.16
Exercise price range, options exercisable (in shares) | shares	603
Exercise price range, options exercisable, weighted-average exercise price (in dollars per share)	$ 75.15
$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	3,441
Exercise price range, options outstanding, weighted-average remaining contractual life (in years)	7 years 8 months 12 days
Exercise price range, options outstanding, weighted-average exercise price (in dollars per share)	$ 89.36
Exercise price range, options exercisable (in shares) | shares	1,336
Exercise price range, options exercisable, weighted-average exercise price (in dollars per share)	$ 90.09
$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	874
Exercise price range, options outstanding, weighted-average remaining contractual life (in years)	6 years 7 months 10 days
Exercise price range, options outstanding, weighted-average exercise price (in dollars per share)	$ 109.28
Exercise price range, options exercisable (in shares) | shares	613
Exercise price range, options exercisable, weighted-average exercise price (in dollars per share)	$ 109.17
$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	1,034
Exercise price range, options outstanding, weighted-average remaining contractual life (in years)	7 years 1 month 17 days
Exercise price range, options outstanding, weighted-average exercise price (in dollars per share)	$ 130.42
Exercise price range, options exercisable (in shares) | shares	702
Exercise price range, options exercisable, weighted-average exercise price (in dollars per share)	$ 130.36
$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	1,500
Exercise price range, options outstanding, weighted-average remaining contractual life (in years)	9 years 7 months 2 days
Exercise price range, options outstanding, weighted-average exercise price (in dollars per share)	$ 155.52
Exercise price range, options exercisable (in shares) | shares	132
Exercise price range, options exercisable, weighted-average exercise price (in dollars per share)	$ 155.02
$160.01–$163.74
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)	$ 163.74
Exercise price range, options outstanding (in shares) | shares	584
Exercise price range, options outstanding, weighted-average remaining contractual life (in years)	9 years 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	114
Exercise price range, options exercisable, weighted-average exercise price (in dollars per share)	$ 162.94