Stock-Based Compensation - Exercise Price for Stock Options Outstanding and Exercisable (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Exercise prices for stock options outstanding and exercisable
Number of stock options outstanding (in shares)	5,490,626
Options Outstanding, Weighted-Average Exercise Price (in dollars per share)	$ 42.81
Stock options
Exercise prices for stock options outstanding and exercisable
Exercise prices, outstanding stock option awards, high end of range (in dollars per share)	$ 80.00
Number of stock options outstanding (in shares)	8,847,734	7,923,009	6,845,685	11,725,823
Options Outstanding, Weighted-Average Remaining Contractual Life	6 years 2 months 12 days
Options Outstanding, Weighted-Average Exercise Price (in dollars per share)	$ 43.90	$ 36.21	$ 31.13	$ 24.51
Number of stock options exercisable	1,772,608	1,892,809	1,965,585
Options Exercisable, Weighted-Average Remaining Contractual Life	4 years 4 months 6 days
Options Exercisable, Weighted-Average Exercise Price (in dollars per share)	$ 35.13	$ 29.97	$ 26.82
Stock options | Range of Exercise Prices $00.00 - $10.00
Exercise prices for stock options outstanding and exercisable
Exercise prices, outstanding stock option awards, high end of range (in dollars per share)	$ 10.00
Number of stock options outstanding (in shares)	133,400
Options Outstanding, Weighted-Average Remaining Contractual Life	1 year 1 month 24 days
Options Outstanding, Weighted-Average Exercise Price (in dollars per share)	$ 6.33
Number of stock options exercisable	133,400
Options Exercisable, Weighted-Average Remaining Contractual Life	1 year 1 month 24 days
Options Exercisable, Weighted-Average Exercise Price (in dollars per share)	$ 6.33
Stock options | Range Of Exercise Prices $10.01 - $20.00
Exercise prices for stock options outstanding and exercisable
Exercise prices, outstanding stock option awards, low end of range (in dollars per share)	10.01
Exercise prices, outstanding stock option awards, high end of range (in dollars per share)	$ 20.00
Number of stock options outstanding (in shares)	1,232,537
Options Outstanding, Weighted-Average Remaining Contractual Life	2 years 5 months 27 days
Options Outstanding, Weighted-Average Exercise Price (in dollars per share)	$ 15.37
Number of stock options exercisable	32,537
Options Exercisable, Weighted-Average Remaining Contractual Life	2 years 3 months
Options Exercisable, Weighted-Average Exercise Price (in dollars per share)	$ 15.13
Stock options | Range of Exercise Prices $20.01 - $30.00
Exercise prices for stock options outstanding and exercisable
Exercise prices, outstanding stock option awards, low end of range (in dollars per share)	20.01
Exercise prices, outstanding stock option awards, high end of range (in dollars per share)	$ 30.00
Number of stock options outstanding (in shares)	1,402,183
Options Outstanding, Weighted-Average Remaining Contractual Life	3 years 8 months 9 days
Options Outstanding, Weighted-Average Exercise Price (in dollars per share)	$ 27.13
Number of stock options exercisable	862,183
Options Exercisable, Weighted-Average Remaining Contractual Life	3 years 7 months 24 days
Options Exercisable, Weighted-Average Exercise Price (in dollars per share)	$ 26.65
Stock options | Range of Exercise Prices $30.01 - $40.00
Exercise prices for stock options outstanding and exercisable
Exercise prices, outstanding stock option awards, low end of range (in dollars per share)	30.01
Exercise prices, outstanding stock option awards, high end of range (in dollars per share)	$ 40.00
Number of stock options outstanding (in shares)	1,219,586
Options Outstanding, Weighted-Average Remaining Contractual Life	4 years 11 months 16 days
Options Outstanding, Weighted-Average Exercise Price (in dollars per share)	$ 36.19
Number of stock options exercisable	257,886
Options Exercisable, Weighted-Average Remaining Contractual Life	4 years 9 months
Options Exercisable, Weighted-Average Exercise Price (in dollars per share)	$ 35.06
Stock options | Range of Exercise Prices $40.01 - $50.00
Exercise prices for stock options outstanding and exercisable
Exercise prices, outstanding stock option awards, low end of range (in dollars per share)	40.01
Exercise prices, outstanding stock option awards, high end of range (in dollars per share)	$ 50.00
Number of stock options outstanding (in shares)	360,800
Options Outstanding, Weighted-Average Remaining Contractual Life	8 years
Options Outstanding, Weighted-Average Exercise Price (in dollars per share)	$ 46.21
Number of stock options exercisable	67,600
Options Exercisable, Weighted-Average Remaining Contractual Life	7 years 4 months 21 days
Options Exercisable, Weighted-Average Exercise Price (in dollars per share)	$ 45.89
Stock options | Range of Exercise Prices $50.01 - $60.00
Exercise prices for stock options outstanding and exercisable
Exercise prices, outstanding stock option awards, low end of range (in dollars per share)	50.01
Exercise prices, outstanding stock option awards, high end of range (in dollars per share)	$ 60.00
Number of stock options outstanding (in shares)	2,910,878
Options Outstanding, Weighted-Average Remaining Contractual Life	8 years 4 months 24 days
Options Outstanding, Weighted-Average Exercise Price (in dollars per share)	$ 57.55
Number of stock options exercisable	228,152
Options Exercisable, Weighted-Average Remaining Contractual Life	6 years 6 months 4 days
Options Exercisable, Weighted-Average Exercise Price (in dollars per share)	$ 57.16
Stock options | Range of Exercise Prices $60.01 - $70.00
Exercise prices for stock options outstanding and exercisable
Exercise prices, outstanding stock option awards, low end of range (in dollars per share)	60.01
Exercise prices, outstanding stock option awards, high end of range (in dollars per share)	$ 70.00
Number of stock options outstanding (in shares)	1,568,350
Options Outstanding, Weighted-Average Remaining Contractual Life	8 years 3 months 26 days
Options Outstanding, Weighted-Average Exercise Price (in dollars per share)	$ 64.26
Number of stock options exercisable	170,850
Options Exercisable, Weighted-Average Remaining Contractual Life	6 years 4 months 13 days
Options Exercisable, Weighted-Average Exercise Price (in dollars per share)	$ 66.26
Stock options | Range of Exercise Prices $70.01 - $80.00
Exercise prices for stock options outstanding and exercisable
Exercise prices, outstanding stock option awards, low end of range (in dollars per share)	70.01
Exercise prices, outstanding stock option awards, high end of range (in dollars per share)	$ 80.00
Number of stock options outstanding (in shares)	20,000
Options Outstanding, Weighted-Average Remaining Contractual Life	2 years
Options Outstanding, Weighted-Average Exercise Price (in dollars per share)	$ 72.89
Number of stock options exercisable	20,000
Options Exercisable, Weighted-Average Remaining Contractual Life	2 years
Options Exercisable, Weighted-Average Exercise Price (in dollars per share)	$ 72.89