Parent Company Statement of Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Condensed Financial Statements, Captions [Line Items]
Other Income									$ 249.1	$ 241.6	$ 257.2
Total Revenue	2,604.8	2,545.6	2,564.5	2,563.1	2,593.1	2,527.9	2,510.6	2,561.6	10,278.0	10,193.2	10,091.0
Income (Loss) Before Income Tax	(704.3)	293.3	338.0	330.2	335.5	326.5	308.1	361.2	257.2	1,331.3	1,292.3
Income Tax Expense (Benefit)									21.8	445.2	439.7
Net Income	(425.4)	205.6	229.8	225.4	225.8	220.8	209.7	229.8	235.4	886.1	852.6
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Dividends from Subsidiaries									800.0	576.0	313.8
Interest from Subsidiaries									0.6	0.6	0.5
Other Income									64.4	56.1	47.8
Total Revenue									865.0	632.7	362.1
Interest and Debt Expense									112.1	112.3	90.2
Other Expenses									57.7	47.6	63.9
Total Expenses									169.8	159.9	154.1
Income (Loss) Before Income Tax									695.2	472.8	208.0
Income Tax Expense (Benefit)									(13.9)	(3.6)	(9.5)
Income Before Equity in Undistributed Earnings (Loss) of Subsidiaries									709.1	476.4	217.5
Equity in Undistibuted Earnings (Loss) of Subsidiaries									(473.7)	409.7	635.1
Net Income									$ 235.4	$ 886.1	$ 852.6