Fair Value Measurements (Narrative) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Oct. 31, 2012
1.875% Convertible Debentures [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible debentures	$ 2.2	$ 2.2
Stated interest rate	1.875%	1.875%	1.875%
1.75% Convertible Debentures [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible debentures	295.6	291.1
Stated interest rate	1.75%	1.75%	1.75%	1.75%
Long-term debt, balance outstanding	396.75	396.75
Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Money market fund	$ 93.3	$ 120.0