Bradley Agreement - Schedule of Deficiency Payments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Contractors [Abstract]
Balance at beginning of year	$ 9.9	$ 11.6	$ 13.3
Deficiency payments made	0.3	0.2	0.1
Deficiency repayment received	(2.3)	(1.9)	(1.8)
Balance at end of year	$ 7.8	$ 9.9	$ 11.6