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 Income And Comprehensive Income [Abstract]
Net income	$ 11.1	$ 88.4	$ 134.7	$ 168.6
Available-for-sale securities:
Net appreciation (depreciation) during the period	1.7	(3.6)	40.0	164.2
Change in other-than-temporary impairment losses recognized in other comprehensive income	2.0	1.6	2.5	7.3
Total available-for-sale securities	3.7	(2.0)	42.5	171.5
Pension and postretirement benefits:
Amortization recognized as net periodic benefit and postretirement cost	2.3	1.5	7.0	4.8
Cumulative foreign currency translation adjustment:
Amount recognized as cumulative foreign currency translation during the period	1.1	(1.9)	1.7	(3.2)
Total other comprehensive income (loss), net of tax	7.1	(2.4)	51.2	173.1
Comprehensive income	$ 18.2	$ 86.0	$ 185.9	$ 341.7