Income Taxes - Income Tax Reconciliation (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
Income (loss) before income taxes	$ 113.2	$ 126.7	$ 58.5	$ 218.2	$ 18.3	$ (34.1)	$ 159.7	$ 171.4	$ 516.6	$ 315.3	$ 486.9
Tax rate									35.00%	35.00%	35.00%
Income tax expense (benefit) at federal statutory rate									180.8	110.4	170.4
Dividends received deduction									(18.6)	(37.0)	(23.3)
Valuation allowance									0	(87.0)	(13.7)
IRS audit adjustment									(0.3)	3.7	(26.8)
Prior year tax									28.1	0	0
State tax expense (benefit)									0	0	0.6
Other									1.2	4.9	1.8
Income tax expense (benefit)									191.2	(5.0)	109.0
Decrease in estimate of certain deferred tax assets									$ 28.1