Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 12.6	$ (11.1)	$ 40.2	$ 40.7
Other comprehensive income (loss), before tax:
Unrealized gains/losses on securities	69.2	37.5	353.7	1,002.4
Other-than-temporary impairments	0.1	0.4	1.9	1.2
Pension and other postretirement benefits liability	(0.6)	(0.5)	(1.7)	(1.6)
Other comprehensive income (loss), before tax	68.7	37.4	353.9	1,002.0
Income tax expense (benefit) related to items of other comprehensive income (loss)	23.9	16.1	122.5	348.5
Other comprehensive income (loss), after tax	44.8	21.3	231.4	653.5
Comprehensive income (loss)	$ 57.4	$ 10.2	$ 271.6	$ 694.2