Reconciliation of Gross Unrealized Investment Gains or losses to Net Balance (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Investments Unrealized Gain Loss Positions [Line Items]
Unrealized gains arising during period	$ 146.2	$ 121.3	$ 139.8
Less: reclassification adjustments for gains realized in net income	29.0	29.7	29.2
Total available-for-sale securities	117.2	91.6	110.6
Unrealized gains (losses) arising during period	0.1	(1.2)
Other comprehensive income	117.3	90.4	110.6
Unrealized gains arising during period	(40.2)	23.5	9.1
Less: reclassification adjustments for gains realized in net income	5.9	9.3	3.2
Total available-for-sale securities	(46.1)	14.2	5.9
Unrealized gains (losses) arising during period	(0.1)	0.6
Other comprehensive income	(46.2)	14.8	5.9
Unrealized gains arising during period	186.4	97.8	130.7
Less: reclassification adjustments for gains realized in net income	23.1	20.4	26.0
Total available-for-sale securities	163.3	77.4	104.7
Unrealized gains (losses) arising during period	0.2	(1.8)
Other comprehensive income	$ 163.5	$ 75.6	$ 104.7