Bradley Agreement - Schedule of Deficiency Payments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Deficiency Payments [Roll Forward]
Balance at beginning of year	$ 7.8	$ 9.9	$ 11.6
Deficiency payments made	0.1	0.3	0.2
Deficiency repayment received	(4.0)	(2.3)	(1.9)
Balance at end of year	$ 3.9	$ 7.8	$ 9.9