Parent Company Statement of Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Condensed Financial Statements, Captions [Line Items]
Other Income									$ 227.9	$ 249.1	$ 241.6
Total Revenue	2,658.2	2,628.0	2,617.9	2,611.3	2,604.8	2,545.6	2,564.5	2,563.1	10,515.4	10,278.0	10,193.2
Income (Loss) Before Income Tax	322.7	320.4	296.4	310.0	(617.3)	288.7	334.5	327.4	1,249.5	333.3	1,319.9
Income Tax Expense (Benefit)									355.1	49.1	441.2
Net Income	233.9	230.2	216.4	213.9	(369.0)	202.0	227.6	223.6	894.4	284.2	878.7
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Dividends from Subsidiaries									670.8	800.0	576.0
Interest from Subsidiaries									0.6	0.6	0.6
Other Income									54.4	64.4	56.1
Total Revenue									725.8	865.0	632.7
Interest and Debt Expense									114.2	112.1	112.3
Other Expenses									65.5	57.7	47.6
Total Expenses									179.7	169.8	159.9
Income (Loss) Before Income Tax									546.1	695.2	472.8
Income Tax Expense (Benefit)									(25.7)	(13.9)	(3.6)
Income Before Equity in Undistributed Earnings (Loss) of Subsidiaries									571.8	709.1	476.4
Equity in Undistibuted Earnings (Loss) of Subsidiaries									322.6	(424.9)	402.3
Net Income									$ 894.4	$ 284.2	$ 878.7