Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 67.7	$ 45.2
Available-for-sale securities:
Net (depreciation) appreciation during the period	(123.9)	18.9
Adoption of Accounting Standards Update 2016-01 and 2018-02	(81.6)
Change in other-than-temporary impairment losses recognized in other comprehensive income (loss)	0.4	0.1
Total available-for-sale securities	(205.1)	19.0
Pension and postretirement benefits:
Net change in net actuarial loss and prior service cost	(2.3)	2.3
Adoption of Accounting Standards Update 2018-02	(16.2)
Total pension and postretirement benefits	(18.5)	2.3
Cumulative foreign currency translation adjustment:
Amount recognized as cumulative foreign currency translation during the period	(0.5)	5.1
Adoption of Accounting Standards Update 2018-02	(3.9)
Total cumulative foreign currency translation adjustment	(4.4)	5.1
Total other comprehensive (loss) income, net of tax	(228.0)	26.4
Comprehensive (loss) income	$ (160.3)	$ 71.6