Debt - Narrative (Details) shares in Millions, $ in Millions	9 Months Ended
	Sep. 30, 2018 item	Sep. 30, 2017 USD ($) shares
Convertible Senior Notes 0.375 Percent Due 2018 and Convertible Senior Notes 1.25 Percent Due 2020
Convertible Notes
Number of days within 30 consecutive trading days in which the price of the entity's common stock must exceed the conversion price for the notes to be converted	20 days
Number of consecutive trading days during which the closing price of the entity's common stock must exceed the conversion price for at least 20 days in order for the notes to be convertible	30 days
Percentage of the closing sales price of common stock that the conversion price must exceed in order for the notes to be convertible (as a percent)	130.00%
Number of consecutive business days immediately after any five consecutive trading day period during the note measurement period | item	5
Number of consecutive trading days before five consecutive business days during the note measurement period | item	5
Conversion ratio, principal amount of note	1,000
Percentage of the trading price to the product of the last reported sale price of the common stock and the conversion rate, maximum (as a percent)	98.00%
Number of scheduled trading days preceding relevant maturity date at which Notes are convertible regardless of other circumstances	2 days
Fair value of premium stock issued and cash paid		$ 48.4
Convertible Senior Notes 0.375 Percent Due 2018 and Convertible Senior Notes 1.25 Percent Due 2020 | Director
Convertible Notes
Common stock issued in exchange of notes (in shares) | shares		10.6
Convertible Senior Notes 0.375 Percent Due 2018
Convertible Notes
Interest rate of debt (as a percent)	0.375%	0.375%
Aggregate principal amount of notes exchanged		$ 367.2
Common stock issued in exchange of notes (in shares) | shares		7.1
Premium stock issued in exchange of notes (in shares) | shares		0.1
Value of premium shares issued		$ 12.6
Cash used to fund redemption of notes		2.0
Debt exchange expense		1.4
Convertible Senior Notes 0.375 Percent Due 2018 | Director
Convertible Notes
Aggregate principal amount of notes exchanged		$ 259.0
Convertible Senior Notes 1.25 Percent Due 2020
Convertible Notes
Interest rate of debt (as a percent)	1.25%	1.25%
Aggregate principal amount of notes exchanged		$ 355.6
Common stock issued in exchange of notes (in shares) | shares		6.9
Premium stock issued in exchange of notes (in shares) | shares		0.2
Value of premium shares issued		$ 26.8
Cash used to fund redemption of notes		7.0
Debt exchange expense		5.1
Convertible Senior Notes 1.25 Percent Due 2020 | Director
Convertible Notes
Aggregate principal amount of notes exchanged		$ 274.5