CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Comprehensive income:
Net income (loss)	$ 132.4	$ 106.5	$ (157.1)	$ 523.9
Defined benefit pension plan:
Actuarial loss arising during period, net of tax			(0.6)
Amortization of actuarial gain and prior service cost included in net periodic pension cost, net of tax	(0.1)	0.4	(1.6)	1.0
Total defined benefit pension plan, net of tax	(0.1)	0.4	(2.2)	1.0
Unrealized gain (loss) on available-for-sale securities, net of tax	1.0	(0.7)	6.0	(3.6)
Other comprehensive income (loss), net of tax	0.9	(0.3)	3.8	(2.6)
Comprehensive income (loss)	$ 133.3	$ 106.2	$ (153.3)	$ 521.3