Stockholders' Equity (Details 3) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	$ 272.4
Net change in noncredit component of impairment losses on fixed maturities, available-for-sale	(49.7)	56.1	260.9
Accumulated other comprehensive income (loss), balance at end of period	201.9	272.4
Adjustments necessary to avoid duplication of items included in net income that had been part of other comprehensive income in prior years
Unrealized gains (losses) on available-for-sale securities and derivative instruments, as reported	245.1	1,073.2	3,530.3
Adjustment for realized gains (losses) on available-for-sale securities and derivative instruments included in net income	(117.8)	(137.7)	(215.8)
Unrealized gains (losses) on available-for-sale securities and derivative instruments arising during the year	127.3	935.5	3,314.5
Dividend Limitations
Statutory dividend limitation as percentage of policyholder surplus (as a percent)	10.00%
Dividends that can be paid in upcoming calendar year without exceeding statutory limitation	507.7
Net unrealized gains (losses) on available-for-sale securities
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	395.4	(654.8)	(4,208.0)
Net change in unrealized gains (losses) on fixed maturities, available-for-sale	650.0	2,315.1	6,590.8
Net change in noncredit component of impairment losses on fixed maturities, available-for-sale	49.9	(56.1)	(260.9)
Net change in unrealized gains (losses) on equity securities, available-for-sale	12.3	7.0	48.1
Net change in unrealized gains (losses) on equity method subsidiaries and noncontrolling interest adjustments	9.1	(68.9)	111.7
Adjustments for assumed changes in amortization patterns	(167.6)	(488.0)	(963.3)
Adjustments for assumed changes in policyholder liabilities	(230.3)	(136.7)	(79.0)
Effects of reclassifying noncredit component of previously recognized impairment losses on fixed maturities, available-for-sale, net			(9.9)
Effects of implementation of accounting change related to variable interest entities, net		10.7
Effects of electing fair value option for fixed maturities upon implementation of accounting change related to embedded credit derivatives, net		25.4
Provision for deferred income tax benefits (taxes)	(117.3)	(558.3)	(1,884.3)
Accumulated other comprehensive income (loss), balance at end of period	601.5	395.4	(654.8)
Net unrealized gains (losses) on derivative instruments
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	37.3	14.3	37.2
Net change in unrealized gains (losses) on derivative instruments	60.2	36.7	(33.2)
Provision for deferred income tax benefits (taxes)	(21.2)	(13.7)	10.3
Accumulated other comprehensive income (loss), balance at end of period	76.3	37.3	14.3
Foreign currency translation adjustment
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	27.9	(5.3)	(173.5)
Change in net foreign currency translation adjustment	(142.7)	20.8	191.8
Provision for deferred income tax benefits (taxes)		12.4	(23.6)
Accumulated other comprehensive income (loss), balance at end of period	(114.8)	27.9	(5.3)
Unrecognized postretirement benefit obligations
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	(188.2)	(396.2)	(567.3)
Change in unrecognized postretirement benefit obligation	(266.0)	320.0	263.3
Provision for deferred income tax benefits (taxes)	93.1	(112.0)	(92.2)
Accumulated other comprehensive income (loss), balance at end of period	(361.1)	(188.2)	(396.2)
Accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss) rollforward
Accumulated other comprehensive income (loss), balance at beginning of period	272.4	(1,042.0)	(4,911.6)
Net change in unrealized gains (losses) on fixed maturities, available-for-sale	650.0	2,315.1	6,590.8
Net change in noncredit component of impairment losses on fixed maturities, available-for-sale	49.9	(56.1)	(260.9)
Net change in unrealized gains (losses) on equity securities, available-for-sale	12.3	7.0	48.1
Net change in unrealized gains (losses) on equity method subsidiaries and noncontrolling interest adjustments	9.1	(68.9)	111.7
Adjustments for assumed changes in amortization patterns	(167.6)	(488.0)	(963.3)
Adjustments for assumed changes in policyholder liabilities	(230.3)	(136.7)	(79.0)
Net change in unrealized gains (losses) on derivative instruments	60.2	36.7	(33.2)
Change in net foreign currency translation adjustment	(142.7)	20.8	191.8
Change in unrecognized postretirement benefit obligation	(266.0)	320.0	263.3
Effects of reclassifying noncredit component of previously recognized impairment losses on fixed maturities, available-for-sale, net			(9.9)
Effects of implementation of accounting change related to variable interest entities, net		10.7
Effects of electing fair value option for fixed maturities upon implementation of accounting change related to embedded credit derivatives, net		25.4
Provision for deferred income tax benefits (taxes)	(45.4)	(671.6)	(1,989.8)
Accumulated other comprehensive income (loss), balance at end of period	$ 201.9	$ 272.4	$ (1,042.0)