Parent Company Notes - Additional Details (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended				12 Months Ended							12 Months Ended		12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2013 Parent Company [Member]	Dec. 31, 2012 Parent Company [Member]	Dec. 31, 2011 Parent Company [Member]	Dec. 31, 2013 Notes 5 Point 750 Due 2042 Callable At or Above Par [Member]	Dec. 31, 2012 Notes 5 Point 750 Due 2042 Callable At or Above Par [Member]	Dec. 31, 2013 Junior Subordinated Debt Securities 7 Point 405 Percent due 2038 [Member]	Dec. 31, 1998 Junior Subordinated Debt Securities 7 Point 405 Percent due 2038 [Member]	Dec. 31, 2013 Notes 6 Point 85 Percent due 2015 callable at or above par [Member]	Dec. 31, 2013 Notes 6 Point 85 Percent due 2015 callable at or above par [Member] Subsidiaries [Member]	Dec. 31, 2011 Notes 7 Point 625 Percent due 2011 callable at or above par, current portion [Member]	Dec. 31, 2011 Notes 7 Point 625 Percent due 2011 callable at or above par, current portion [Member] Medium-term Notes 7 Point 0 Percent to 7 Point 2 Percent due 2023 to 2028 noncallable [Member]	Dec. 31, 2013 Notes 7 Point 625 Percent due 2011 callable at or above par, current portion [Member] Medium-term Notes 7 Point 0 Percent to 7 Point 2 Percent due 2023 to 2028 noncallable [Member]	Dec. 31, 2013 Receive Fixed/Pay Variable Interest Rate Swaps	Dec. 31, 2012 Receive Fixed/Pay Variable Interest Rate Swaps
Long-term Debt, Maturities, Repayments of Principal in Year Three	$ 350.0				$ 350.0
Long-term Debt, Maturities, Repayments of Principal in Year Five	200.0				200.0
Long-term Debt, Maturities, Repayments of Principal after Year Five	1,772.6				1,332.6
Proceeds from Unsecured Notes Payable								250.0	250.0
Derivative, Notional Amount	1,477.4	1,432.8	1,413.0	1,681.9													600.0	600.0
Junior Subordinated Notes										300.0	300.0
Debt Instrument, Interest Rate, Stated Percentage										7.405%	7.405%	6.85%		7.625%		7.625%
Liquidation value per capital security										$ 1,000	$ 1,000
Repayments of Medium-term Notes														225.1	225.1
Interest Paid	144.6	139.6	145.4		116.5	109.0	115.6
Line of Credit Facility Original Expiration Period	5 years
Line of Credit Facility, Maximum Borrowing Capacity	400.0
Line Of Credit Facility Maximum Borrowing Capacity If Requested	600.0
Line of Credit Facility, Amount Outstanding	0
Guarantor Obligations, Liquidation Proceeds, Monetary Amount												400.0
Long-term Debt and Capital Lease Obligations	$ 2,612.0	$ 2,755.4			$ 1,875.2	$ 1,896.2							$ 296.9