Parent Company Statement of Income (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Condensed Financial Statements, Captions
Other Income									$ 211.5	$ 219.0	$ 230.2
Total Revenue	$ 2,722.4	$ 2,657.8	$ 2,703.7	$ 2,647.4	$ 2,639.0	$ 2,614.3	$ 2,656.8	$ 2,614.4	10,731.3	10,524.5	10,368.6
Income Before Income Tax	316.6	298.9	319.8	303.0	(454.6)	317.0	349.8	329.8	1,238.3	542.0	1,220.0
Income Tax Expense (Benefit)									371.2	139.9	373.0
Net Income	$ 226.1	$ 203.8	$ 224.3	$ 212.9	$ (282.2)	$ 219.1	$ 239.4	$ 225.8	867.1	402.1	847.0
Net Other Comprehensive Income (Loss)									(150.3)	(88.6)	(373.0)
Comprehensive Income									716.8	313.5	474.0
Parent Company
Condensed Financial Statements, Captions
Cash Dividends from Subsidiaries									671.8	645.2	636.6
Other Income									52.8	116.3	56.9
Total Revenue									724.6	761.5	693.5
Interest and Debt Expense									137.5	132.4	120.9
Other Expenses									16.7	66.0	48.9
Total Expenses									154.2	198.4	169.8
Income Before Income Tax									570.4	563.1	523.7
Income Tax Expense (Benefit)									(16.2)	(24.5)	(14.7)
Income of Parent Company									586.6	587.6	538.4
Equity in Undistibuted Earnings (Loss) of Subsidiaries									280.5	(185.5)	308.6
Net Income									867.1	402.1	847.0
Net Other Comprehensive Income (Loss)									(150.3)	(88.6)	(373.0)
Comprehensive Income									$ 716.8	$ 313.5	$ 474.0