Financial Instruments (Realized Gains and Losses) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Fair Value Disclosures [Abstract]
Proceeds from sales	$ 362.9	$ 4,953.8	$ 456.6	$ 5,546.4
Realized gross gains on sales	13.0	5.6	15.5	7.7
Realized gross losses on sales	$ 6.4	$ 47.4	$ 6.8	$ 49.0