Parent Company Statement of Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Other Income									$ 219.0	$ 230.2	$ 227.9
Total Revenue	2,635.7	2,609.4	2,653.5	2,611.1	2,586.2	2,540.9	2,601.9	2,624.8	10,509.7	10,353.8	10,515.4
Income Before Income Tax	(457.9)	312.1	346.5	326.5	305.8	284.1	311.5	303.8	527.2	1,205.2	1,249.5
Income Tax Expense (Benefit)									113.8	347.1	355.1
Net Income	(279.1)	221.1	242.5	228.9	221.2	205.7	218.6	212.6	413.4	858.1	894.4
Total Other Comprehensive Income (Loss)									(88.6)	(373.0)	166.2
Comprehensive Income									324.8	485.1	1,060.6
Parent Company
Condensed Financial Statements, Captions [Line Items]
Cash Dividends from Subsidiaries									645.2	636.6	670.8
Other Income									116.3	56.9	55.0
Total Revenue									761.5	693.5	725.8
Interest and Debt Expense									132.4	120.9	114.2
Other Expenses									66.0	48.9	65.5
Total Expenses									198.4	169.8	179.7
Income Before Income Tax									563.1	523.7	546.1
Income Tax Expense (Benefit)									(24.5)	(14.7)	(25.7)
Income of Parent Company									587.6	538.4	571.8
Equity in Undistibuted Earnings (Loss) of Subsidiaries									(174.2)	319.7	322.6
Net Income									413.4	858.1	894.4
Total Other Comprehensive Income (Loss)									(88.6)	(373.0)	166.2
Comprehensive Income									$ 324.8	$ 485.1	$ 1,060.6