Stock-Based Compensation - Options outstanding (Details) - Stock options shares in Thousands	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Options Outstanding
Number Outstanding (in shares) | shares	94,544
Weighted Average Remaining Contractual Life	5 years 10 months 17 days
Weighted Average Exercise Price (in dollars per share)	$ 54.91
Options Exercisable
Number Exercisable (in shares) | shares	93,342
Weighted Average Exercise Price (in dollars per share)	$ 54.85
$29.05–$37.41
Information about options outstanding
Low end of exercise price range (in dollars per share)	29.05
High end of exercise price range (in dollars per share)	$ 37.41
Options Outstanding
Number Outstanding (in shares) | shares	7,150
Weighted Average Remaining Contractual Life	4 years 5 months 12 days
Weighted Average Exercise Price (in dollars per share)	$ 36.24
Options Exercisable
Number Exercisable (in shares) | shares	7,150
Weighted Average Exercise Price (in dollars per share)	$ 36.24
$54.76–$54.76
Information about options outstanding
Low end of exercise price range (in dollars per share)	54.76
High end of exercise price range (in dollars per share)	$ 54.76
Options Outstanding
Number Outstanding (in shares) | shares	39,781
Weighted Average Remaining Contractual Life	5 years 7 months 2 days
Weighted Average Exercise Price (in dollars per share)	$ 54.76
Options Exercisable
Number Exercisable (in shares) | shares	39,781
Weighted Average Exercise Price (in dollars per share)	$ 54.76
$57.47–$60.10
Information about options outstanding
Low end of exercise price range (in dollars per share)	57.47
High end of exercise price range (in dollars per share)	$ 60.10
Options Outstanding
Number Outstanding (in shares) | shares	47,613
Weighted Average Remaining Contractual Life	6 years 4 months 6 days
Weighted Average Exercise Price (in dollars per share)	$ 57.84
Options Exercisable
Number Exercisable (in shares) | shares	46,411
Weighted Average Exercise Price (in dollars per share)	$ 57.78