Fair Value of Financial Instruments and Hedging Activities - Carrying Value and Fair Value of Other Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity investments	$ 133.4	$ 118.7
Senior Notes | 2.900% Senior Notes Due 2024
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	599.2	599.2
Senior Notes | 2.900% Senior Notes Due 2024 | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	599.2	599.2
Senior Notes | 3.450% Senior Notes Due 2029
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	598.3	598.2
Senior Notes | 3.450% Senior Notes Due 2029 | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	598.3	598.2
Senior Notes | 1.450% Senior Notes due 2027
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	549.3	0.0
Senior Notes | 1.450% Senior Notes due 2027 | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	549.3	0.0
Level 1 | Senior Notes | 2.900% Senior Notes Due 2024 | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	580.1	602.1
Level 1 | Senior Notes | 3.450% Senior Notes Due 2029 | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	536.3	603.1
Level 1 | Senior Notes | 1.450% Senior Notes due 2027 | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	478.9	0.0
Level 2 | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity investments	133.4	118.7
GDS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity investments	$ 133.4	$ 118.7