Condensed Consolidated Statements of Comprehensive Income (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Net income	$ 144.0	$ 87.4
Other Comprehensive Income (Loss), before Tax [Abstract]
Change in unrealized gains on securities	(72.4)	(16.1)
Change in other-than-temporary impairment losses	1.6	5.3
Pension and other post-employment benefit liability	(0.6)	0
Other comprehensive income, before tax	(71.4)	(10.8)
Income tax expense related to items of other comprehensive income	31.1	(1.9)
Other comprehensive income, after tax	(40.3)	(12.7)
Total comprehensive income	$ 103.7	$ 74.7