Nature of Operations and Significant Accounting Policies (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended			3 Months Ended						3 Months Ended		3 Months Ended		3 Months Ended		12 Months Ended		3 Months Ended		12 Months Ended		3 Months Ended		12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011	Mar. 31, 2012 Retained earnings (deficit)	Mar. 31, 2011 Retained earnings (deficit)	Mar. 31, 2012 Accumulated other comprehensive income (loss)	Dec. 31, 2011 Accumulated other comprehensive income (loss)	Mar. 31, 2011 Accumulated other comprehensive income (loss)	Dec. 31, 2010 Accumulated other comprehensive income (loss)	Mar. 31, 2011 As originally reported	Dec. 31, 2011 As originally reported	Mar. 31, 2011 Effect of change	Dec. 31, 2011 Effect of change	Mar. 31, 2012 Former reinsurance accounting method	Mar. 31, 2012 Effect of reinsurance accounting change	Dec. 31, 2010 Combined effects of implementation of accounting change related to deferred policy acquisition costs and reinsurance accounting change Retained earnings (deficit)	Dec. 31, 2010 Combined effects of implementation of accounting change related to deferred policy acquisition costs and reinsurance accounting change Accumulated other comprehensive income (loss)	Mar. 31, 2011 Effects of implementation of accounting change related to deferred policy acquisition costs	Dec. 31, 2011 Effects of implementation of accounting change related to deferred policy acquisition costs	Dec. 31, 2010 Effects of implementation of accounting change related to deferred policy acquisition costs Retained earnings (deficit)	Dec. 31, 2010 Effects of implementation of accounting change related to deferred policy acquisition costs Accumulated other comprehensive income (loss)	Mar. 31, 2011 Effects of reinsurance accounting change	Dec. 31, 2011 Effects of reinsurance accounting change	Dec. 31, 2010 Effects of reinsurance accounting change Retained earnings (deficit)	Dec. 31, 2010 Effects of reinsurance accounting change Accumulated other comprehensive income (loss)
Recent Accounting Pronouncements and Accounting Changes Disclosures
Cumulative effect of adoption of new accounting principle, increase (decrease) in equity account																$ (612.9)	$ 34.3			$ (631.7)	$ 29.5			$ 18.8	$ 4.8
Assets
Other investments	2,952.5		2,985.8								2,988.0		(2.2)						(2.2)
Premiums due and other receivables	1,118.8		1,196.5								1,245.2		(48.7)	1,182.6	(63.8)								(48.7)
Deferred policy acquisition costs	2,665.6		2,428.0								3,313.5		(885.5)	2,648.7	16.9				(884.4)				(1.1)
Liabilities
Future policy benefits and claims	20,604.0		20,210.4								20,207.9		2.5	20,603.9	0.1								2.5
Other policyholder funds	639.1		548.6								543.7		4.9	632.6	6.5				7.0				(2.1)
Deferred income taxes	492.8		208.7								533.4		(324.7)	511.5	(18.7)				(307.1)				(17.6)
Stockholders' equity
Retained earnings (deficit)	4,548.5		4,402.3								5,077.5		(675.2)	4,581.9	(33.4)				(642.0)				(33.2)
Accumulated other comprehensive income (loss)	488.1		258.0			488.1	258.0	566.0	306.7		201.9		56.1	489.5	(1.4)				55.5				0.6
Revenues
Fees and other revenues	598.0	623.0								620.8		2.2		606.5	(8.5)			0.1				2.1
Net investment income (loss)	824.8	859.8								859.9		(0.1)						(0.1)
Expenses
Benefits, claims and settlement expenses	1,212.5	1,188.9								1,191.5		(2.6)		1,199.8	12.7							(2.6)
Operating expenses	556.0	717.9								691.2		26.7		576.9	(20.9)			20.1				6.6
Income (loss) before income taxes	277.1	261.5								283.5		(22.0)		277.4	(0.3)			(20.1)				(1.9)
Income taxes (benefits)	58.2	52.7								60.4		(7.7)		58.3	(0.1)			(7.0)				(0.7)
Net income (loss)	218.9	208.8		209.7	190.2					223.1		(14.3)		219.1	(0.2)			(13.1)				(1.2)
Net income (loss) available to common stockholders	$ 201.5	$ 182.0								$ 196.3		$ (14.3)		$ 201.7	$ (0.2)			$ (13.1)				$ (1.2)
Basic earnings per common share (in dollars per share)	$ 0.67	$ 0.57								$ 0.61		$ (0.04)		$ 0.67				$ (0.04)
Diluted earnings per common share (in dollars per share)	$ 0.66	$ 0.56								$ 0.60		$ (0.04)		$ 0.66				$ (0.04)