Parent Company Notes - Additional Details (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 1998	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2012
Repayments of Principal in Next Twelve Months						$ 350.0
Repayments of Principal in 2018						200.0
Repayments of Principal in 2020						400.0
Repayments of Principal Thereafter						1,881.6
Notional Amount of Derivatives		$ 1,687.4	$ 1,477.4			1,470.3		$ 1,432.8
Interest Paid	$ 146.9	145.9	144.6
Line of Credit Facility, Maximum Borrowing Capacity						400.0
Line Of Credit Facility Maximum Borrowing Capacity If Requested						600.0
Letters of Credit Outstanding		2.1				2.1
Amount Outstanding on Line of Credit		0.0				0.0
Parent Company
Repayments of Principal in Next Twelve Months						350.0
Repayments of Principal in 2018						200.0
Repayments of Principal in 2020						400.0
Repayments of Principal Thereafter						1,557.6
Interest Paid	$ 130.9	123.6	$ 116.5
Line of Credit Facility, Maximum Borrowing Capacity						400.0
Line Of Credit Facility Maximum Borrowing Capacity If Requested						600.0
Letters of Credit Outstanding		2.1				2.1
Amount Outstanding on Line of Credit		0.0				0.0
Subsidiaries
Stated Interest Rate of Debt	6.85%
Guarantor Obligations, Liquidation Proceeds, Monetary Amount	$ 400.0
Receive Fixed/Pay Variable | Swaps
Notional Amount of Derivatives		600.0				600.0
Receive Fixed/Pay Variable | Swaps | Parent Company
Notional Amount of Derivatives		$ 600.0				$ 600.0
Notes Issued in 1998 with 2038 Maturity Date | Junior Subordinated Debt
Proceeds from Issuance of Senior Long-term Debt				$ 300.0
Stated Interest Rate of Debt	7.405%						7.405%
Liquidation value per capital security					$ 1,000
Notes Issued in 1998 with 2038 Maturity Date | Junior Subordinated Debt | Parent Company
Proceeds from Issuance of Senior Long-term Debt				$ 300.0
Stated Interest Rate of Debt	7.405%						7.405%
Liquidation value per capital security					$ 1,000