Rollforward of Cumulative Amounts Related to Credit Loss Portion of Other than Temporary Impairments Losses (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule of Investments [Line Items]
Credit losses as of the beginning of the year	$ 14.5	$ 16.7	$ 20.0
Credit losses for which an OTTI was not previously recognized	0.6		1.2
Additional credit losses on securities for which an OTTI was previously recognized	0.6	0.9	3.1
Reductions for securities sold, matured or called	(6.9)	(1.8)	(7.2)
Reductions for securities reclassified as intend to sell	(0.2)	(1.3)	(0.4)
Credit losses as of the end of the year	$ 8.6	$ 14.5	$ 16.7

The cumulative amount of credit losses recognized in earnings related to debt securities held for which a portion of an other than temporary impairment (OTTI) was recognized in other comprehensive income (a component of shareholders' equity).

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This item represents reductions in the amount of cumulative losses recognized in earnings for other than temporary impairments (OTTI) for which a portion of the impairment had been recognized in other comprehensive income (a component of shareholders' equity) due to a change in status of the impaired security because of a change in the entity's intent to either sell the impaired security or it has become more likely than not that the entity will be required to sell the security before recovery of its cost basis.

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