Consolidated Statements of Comprehensive Income (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 82.6	$ 53.4	$ 137.2	$ 119.6
Available-for-sale securities and derivative instruments:
Net appreciation (depreciation) during the period	34.3	(138.2)	81.5	(132.7)
Change in other-than-temporary impairment losses recognized in other comprehensive income		0.8	1.6	0.7
Total available-for-sale securities and derivative instruments	34.3	(137.4)	83.1	(132.0)
Pension and postretirement benefits:
Amortization recognized as net periodic benefit and postretirement cost	1.6	4.1	3.4	6.0
Cumulative foreign currency translation adjustment:
Amount recognized as cumulative foreign currency translation during the period	3.1	(0.1)	3.0	(10.1)
Total other comprehensive income (loss), net of tax	39.0	(133.4)	89.5	(136.1)
Comprehensive income (loss)	$ 121.6	$ (80.0)	$ 226.7	$ (16.5)