Stock-Based Compensation - Stock Option Activity (Details) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
Options, Outstanding, Shares
Options, Beginning balance (in shares) | shares	3.8
Shares, exercised (in shares) | shares	(1.7)
Shares, cancelled, forfeited and expired (in shares) | shares	0.0
Options, Ending balance (in shares) | shares	2.1
Weighted average exercise price
Weighted-average exercise price, Beginning balance (in dollars per share) | $ / shares	$ 43.36
Weighted-average exercise price, exercised (in dollars per share) | $ / shares	113.04
Weighted-average exercise price, cancelled, forfeited and expired (in dollars per share) | $ / shares	72.35
Weighted-average exercise price, Ending balance (in dollars per share) | $ / shares	$ 44.09
Shares, options exercisable (in shares) | shares	2.1
Weighted-average exercise price, options exercisable (in dollars per share) | $ / shares	$ 44.08
Weighted-average remaining years of contractual term, Options outstanding	3 years 6 months
Weighted-average remaining years of contractual term, Options exercisable	3 years 6 months
Aggregate intrinsic value, options outstanding | $	$ 270
Aggregate intrinsic value, options exercisable | $	$ 270
Non Vested Options
Nonvested Options Outstanding
Options, Beginning balance (in shares) | shares	0.2
Shares, vested (in shares) | shares	(0.2)
Shares, forfeited (in shares) | shares	0.0
Options, Ending balance (in shares) | shares	0.0
Weighted-average grant-date fair value
Weighted-average grant-date fair value, Beginning balance (in dollars per share) | $ / shares	$ 23.42
Weighted-average grant-date fair value, vested (in dollars per share) | $ / shares	23.40
Weighted-average exercise price, forfeited (in dollars per share) | $ / shares	24.22
Weighted-average grant-date fair value, Beginning balance (in dollars per share) | $ / shares	$ 27.52
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Total unrecognized compensation expense related to nonvested options | $	$ 0
Weighted-average years to be recognized over	27 days