Business, Basis of Presentation and Significant Accounting Policies - Effects of Changes in Accounting Principles (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 segment	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Accounting Policies [Abstract]
Number of operating segments	1
Retained Earnings Adjustments [Line Items]
Fee income	$ 648.8	$ 614.0	$ 583.5
Other net realized capital gains (losses)	70.2	(108.5)	128.3
Other revenue	0	14.5	33.3
Interest credited and other benefits to contract owners	746.7	763.4	769.2
Retained earnings (deficit)	(4,261.4)	(3,976.3)	(3,092.0)	(2,542.1)
Accounting change to employee benefit plans to recognize actuarial gains and losses immediately in net income
Retained Earnings Adjustments [Line Items]
Change in net income, due to change in accounting principal	1.0	(7.2)	(3.8)
Accumulated Other Comprehensive Income (Loss)
Retained Earnings Adjustments [Line Items]
Retained earnings (deficit)	(1,023.0)	(747.5)	(390.5)	(16.3)
Accumulated Other Comprehensive Income (Loss) | Accounting change to employee benefit plans to recognize actuarial gains and losses immediately in net income
Retained Earnings Adjustments [Line Items]
Effect of change in accounting, adoption of new pronouncement	27.4	28.4		17.4
Retained Earnings (Deficit)
Retained Earnings Adjustments [Line Items]
Retained earnings (deficit)	981.6	1,307.0	1,627.3	2,005.2
Retained Earnings (Deficit) | Accounting change to employee benefit plans to recognize actuarial gains and losses immediately in net income
Retained Earnings Adjustments [Line Items]
Effect of change in accounting, adoption of new pronouncement	(27.4)	(28.4)		(17.4)
Cumulative effect of changes in accounting
Retained Earnings Adjustments [Line Items]
Fee income		(1.1)	(6.3)
Other net realized capital gains (losses)		(216.1)	9.3
Other revenue		(6.0)	(1.4)
Interest credited and other benefits to contract owners		(223.2)	1.6
Cumulative effect of changes in accounting | Accounting change to employee benefit plans to recognize actuarial gains and losses immediately in net income
Retained Earnings Adjustments [Line Items]
Retained earnings (deficit)				0
Cumulative effect of changes in accounting | Deferred policy acquisition costs
Retained Earnings Adjustments [Line Items]
Retained earnings (deficit)				362.0
Cumulative effect of changes in accounting | Accumulated Other Comprehensive Income (Loss) | Accounting change to employee benefit plans to recognize actuarial gains and losses immediately in net income
Retained Earnings Adjustments [Line Items]
Retained earnings (deficit)				(17.4)
Cumulative effect of changes in accounting | Accumulated Other Comprehensive Income (Loss) | Deferred policy acquisition costs
Retained Earnings Adjustments [Line Items]
Retained earnings (deficit)				(13.9)
Cumulative effect of changes in accounting | Retained Earnings (Deficit) | Accounting change to employee benefit plans to recognize actuarial gains and losses immediately in net income
Retained Earnings Adjustments [Line Items]
Retained earnings (deficit)				17.4
Cumulative effect of changes in accounting | Retained Earnings (Deficit) | Deferred policy acquisition costs
Retained Earnings Adjustments [Line Items]
Retained earnings (deficit)				375.9
Before change in method
Retained Earnings Adjustments [Line Items]
Retained earnings (deficit)				(2,904.1)
Before change in method | Deferred policy acquisition costs
Retained Earnings Adjustments [Line Items]
Effect of change in accounting, adoption of new pronouncement, taxes				202.4
Before change in method | Accumulated Other Comprehensive Income (Loss)
Retained Earnings Adjustments [Line Items]
Retained earnings (deficit)				15.0
Before change in method | Retained Earnings (Deficit)
Retained Earnings Adjustments [Line Items]
Retained earnings (deficit)				$ 1,611.9