Investments - Available for Sale Securities (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Net unrealized investment gains (losses):
Equity securities, at cost less impairments	$ 6.4	$ 6.4	$ 15.5	$ 16.5
Gross unrealized gains	9.3	9.3	1.5	21.7
Gross unrealized losses	(0.8)	(0.8)	0.0	0.0
Estimated fair value of equity securities	14.9	14.9	17.0	38.2
Other than temporary impairment losses, investments, portion recognized in earnings, net, available-for-sale securities	$ 10.7	$ 10.7		1.8
Available-for-sale securities, gross unrealized gain			$ 1.6	$ 1.0