Common Stock, Preferred Stock and Equity Plans - Restricted Stock and Restricted Stock Units and PSUs (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restricted Stock
Restricted stock and Restricted Stock Units
Beginning of the period (in shares)	2,613
Granted (in shares)	0
Vested (in shares)	(1,206)
Cancelled (in shares)	(178)
End of the period (in shares)	1,229	2,613
Restricted stock and Restricted Stock Units, weighted-average grant-date fair value
Weighted-average grant-date fair value, as of the beginning of the period (usd per share)	$ 102.54
Weighted-average grant-date fair value, granted (usd per share)	0.00
Weighted-average grant-date fair value, vested (usd per share)	102.99
Weighted-average grant-date fair value, cancelled (usd per share)	102.45
Weighted-average grant-date fair value, as of the end of the period (usd per share)	$ 102.12	$ 102.54
Restricted stock vested in period, total fair value	$ 157.0	$ 74.1	$ 124.0
Restricted Stock Units (excluding PSUs)
Restricted stock and Restricted Stock Units
Beginning of the period (in shares)	798
Granted (in shares)	1,719
Vested (in shares)	(278)
Cancelled (in shares)	(228)
End of the period (in shares)	2,011	798
Restricted stock and Restricted Stock Units, weighted-average grant-date fair value
Weighted-average grant-date fair value, as of the beginning of the period (usd per share)	$ 92.62
Weighted-average grant-date fair value, granted (usd per share)	113.13
Weighted-average grant-date fair value, vested (usd per share)	94.72
Weighted-average grant-date fair value, cancelled (usd per share)	97.86
Weighted-average grant-date fair value, as of the end of the period (usd per share)	$ 109.27	$ 92.62
Restricted stock vested in period, total fair value	$ 33.2	$ 5.3	$ 8.0
Performance-based RSUs
Restricted stock and Restricted Stock Units
Beginning of the period (in shares)	135
Granted (in shares)	392
Vested (in shares)	(15)
Cancelled (in shares)	(28)
End of the period (in shares)	484	135
Restricted stock and Restricted Stock Units, weighted-average grant-date fair value
Weighted-average grant-date fair value, as of the beginning of the period (usd per share)	$ 91.05
Weighted-average grant-date fair value, granted (usd per share)	86.71
Weighted-average grant-date fair value, vested (usd per share)	91.05
Weighted-average grant-date fair value, cancelled (usd per share)	86.52
Weighted-average grant-date fair value, as of the end of the period (usd per share)	$ 87.59	$ 91.05
Restricted stock vested in period, total fair value	$ 1.3
Potential awards, percent of target shares, minimum	0.00%
Potential awards, percent of target shares, maximum	200.00%
Performance-based RSUs | Tranche one
Restricted stock and Restricted Stock Units, weighted-average grant-date fair value
Percent of awards in tranche	50.00%
Performance period	1 year
Vesting period	3 years
Performance-based RSUs | Tranche two
Restricted stock and Restricted Stock Units, weighted-average grant-date fair value
Percent of awards in tranche	50.00%
Performance period	3 years
Vesting period	3 years