Share-Based Compensation Restricted Stock (Details) - Restricted Stock Units (RSUs) $ / shares in Units, shares in Thousands, $ in Millions	6 Months Ended
	Jun. 30, 2017 USD ($) $ / shares shares
Equity stock-based compensation
Vesting terms	3 years
Restricted unit activity
Non-vested at the beginning of the period (in shares) | shares	429
Granted (in shares) | shares	118
Vested (in shares) | shares	(146)
Cancelled (in shares) | shares	(11)
Non-vested at the end of the period (in shares) | shares	390
Weighted-average fair value per share
Non-vested at the beginning of the period (in dollars per share) | $ / shares	$ 81.04
Granted (in dollars per share) | $ / shares	117.46
Vested (in dollars per share) | $ / shares	64.82
Cancelled (in dollars per share) | $ / shares	91.68
Non-vested at the end of the period (in dollars per share) | $ / shares	$ 99.04
Unrecognized compensation cost | $	$ 20
Weighted-average period for amortization of unrecognized compensation cost	1 year 10 months 24 days
Other disclosures
Service period over which compensation expense would be amortized	1 year