Income Taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Effective tax rate on income before income taxes (percentage)	1.90%	1.10%
Net deferred tax assets	$ 112.2		$ 98.6
Income taxes paid	1.3	2.8
Federal excise taxes incurred	$ 2.0	$ 2.0