Income Taxes - Income Tax Reconciliation (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
Income (loss) before income taxes	$ 554.8	$ 420.0	$ 1,276.1	$ 130.5
Tax rate	35.00%	35.00%	35.00%	35.00%
Income tax expense (benefit) at federal statutory rate	194.2	147.0	446.6	45.7
Valuation allowance	(96.7)	(68.3)	(208.7)	94.8
Dividends received deduction	(22.7)	(43.4)	(73.6)	(93.3)
Audit settlement	(0.9)	(0.2)	(1.7)	(1.9)
State tax expense (benefit)	0.5	(5.6)	9.6	(2.3)
Noncontrolling interest	(40.8)	(35.4)	(103.8)	(29.6)
Tax credits	(1.1)	(3.6)	(1.1)	(12.8)
Non-deductible expenses	2.6	(19.3)	3.5	(8.9)
Other	2.3	1.1	3.4	1.9
Income tax expense (benefit)	37.4	(27.7)	74.2	(6.4)
Valuation allowance, deferred tax assets	(2,600.0)		(2,600.0)		(2,800.0)
Allocated to continuing operations
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
Valuation allowance, deferred tax assets	(3,000.0)		(3,000.0)		(3,200.0)
Valuation allowance, deferred tax asset, change in amount	(96.7)	(68.3)	(208.7)	94.8
Allocated to other comprehensive income
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
Valuation allowance, deferred tax assets	(354.1)		(354.1)		(354.1)
Valuation allowance, deferred tax asset, change in amount	$ 0	$ 0	$ 0	$ 0