Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 122.4	$ 67.7
Available-for-sale securities:
Net appreciation (depreciation) during the period	115.0	(123.9)
Change in other-than-temporary impairment losses recognized in other comprehensive income	1.4	0.4
Total available-for-sale securities	116.4	(123.5)
Pension and postretirement benefits:
Net change in net actuarial loss and prior service cost	2.3	(2.3)
Cumulative foreign currency translation adjustment:
Amount recognized as cumulative foreign currency translation during the period	0.1	(0.5)
Total other comprehensive income (loss), net of tax	118.8	(126.3)
Comprehensive income (loss)	$ 241.2	$ (58.6)