Parent Company Statement of Income (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions
Other Income									$ 205.6	$ 211.5	$ 219.0
Total Revenue	$ 2,796.5	$ 2,763.3	$ 2,761.3	$ 2,725.4	$ 2,722.4	$ 2,657.8	$ 2,703.7	$ 2,647.4	11,046.5	10,731.3	10,524.5
Income Before Income Tax	371.2	332.1	340.2	304.2	316.6	298.9	319.8	303.0	1,347.7	1,238.3	542.0
Income Tax Expense (Benefit)									416.3	371.2	139.9
Net Income	$ 248.0	$ 236.0	$ 236.8	$ 210.6	$ 226.1	$ 203.8	$ 224.3	$ 212.9	931.4	867.1	402.1
Net Other Comprehensive Income (Loss)									(67.1)	(150.3)	(88.6)
Comprehensive Income									864.3	716.8	313.5
Parent Company
Condensed Financial Statements, Captions
Cash Dividends from Subsidiaries									676.5	671.8	645.2
Other Income									58.4	52.8	116.3
Total Revenue									734.9	724.6	761.5
Interest and Debt Expense									160.1	137.5	132.4
Other Expenses									32.3	16.7	66.0
Total Expenses									192.4	154.2	198.4
Income Before Income Tax									542.5	570.4	563.1
Income Tax Expense (Benefit)									(20.2)	(16.2)	(24.5)
Income of Parent Company									562.7	586.6	587.6
Equity in Undistibuted Earnings (Loss) of Subsidiaries									368.7	280.5	(185.5)
Net Income									931.4	867.1	402.1
Net Other Comprehensive Income (Loss)									(67.1)	(150.3)	(88.6)
Comprehensive Income									$ 864.3	$ 716.8	$ 313.5