Investments - Current and Non-current of Available-for-Sale (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Reported under the following captions on the consolidated balance sheet:
Total	$ 1,222.8	$ 1,146.2
Cash and cash equivalents
Reported under the following captions on the consolidated balance sheet:
Total	5.9
Current marketable investments
Reported under the following captions on the consolidated balance sheet:
Total	806.4	705.8
Non-current marketable investments
Reported under the following captions on the consolidated balance sheet:
Total	$ 410.5	$ 440.4