Stockholders' Equity - Stock Options Exercise Data (Details) - Stock Options - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Summary of stock option exercise data
Number of options exercised (in shares)	53,911	73,436	387,976	154,541
Cash received	$ 3.6	$ 2.4	$ 36.6	$ 5.0
Total intrinsic value of options exercised	$ 3.2	$ 5.8	$ 23.5	$ 13.4

Aggregate proceeds received by the entity during the annual period from exercises of stock or unit options and conversion of similar instruments granted under equity-based payment arrangements.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

+ References

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Number of share options (or share units) exercised during the current period.

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