Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 2.0	$ 120.7	$ 80.2	$ 175.6
Available-for-sale securities:
Net appreciation (depreciation) during the period	74.2	(92.6)	167.8	(63.5)
Change in other-than-temporary impairment losses recognized in other comprehensive income	2.8	(2.1)	5.7	(4.4)
Total available-for-sale securities	77.0	(94.7)	173.5	(67.9)
Pension and postretirement benefits:
Net actuarial losses and prior service cost arising in the period, Net of Tax		(3.3)		(1.4)
Amortization recognized as net periodic benefit and postretirement cost	1.7	3.4	3.3	5.1
Total pension and postretirement benefits	1.7	0.1	3.3	3.7
Cumulative foreign currency translation adjustment:
Amount recognized as cumulative foreign currency translation during the period	(0.7)	7.7	(1.3)	(2.2)
Total other comprehensive income (loss), net of tax	78.0	(86.9)	175.5	(66.4)
Comprehensive income	$ 80.0	$ 33.8	$ 255.7	$ 109.2