Share-Based Compensation Restricted Stock (Details) - Restricted Stock Units (RSUs) $ / shares in Units, shares in Thousands, $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($) $ / shares shares
Stock Options
Vesting terms	3 years
Service period over which compensation expense would be amortized	1 year
Restricted stock unit activity
Non-vested at the beginning of the period (in shares) | shares	387
Granted (in shares) | shares	106
Vested (in shares) | shares	(124)
Cancelled (in shares) | shares	(10)
Non-vested at the end of the period (in shares) | shares	359
Weighted-average fair value per share
Non-vested at the beginning of the period (in dollars per share) | $ / shares	$ 100.13
Granted (in dollars per share) | $ / shares	131.00
Vested (in dollars per share) | $ / shares	83.37
Cancelled (in dollars per share) | $ / shares	112.90
Non-vested at the end of the period (in dollars per share) | $ / shares	$ 114.57
Unrecognized compensation cost | $	$ 20
Weighted-average period for amortization of unrecognized compensation cost	2 years