Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Parent Company Notes - Additional Details (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 1998	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2015
Repayments of Principal in 2020					$ 400.0
Repayments of Principal in 2021					350.0
Repayments of Principal in 2023					2.0
Repayments of Principal Thereafter					2,245.6
Current Maturities of Long-term Debt		$ 199.9			0.0
Long-term Debt, Excluding Current Maturities		2,738.4			2,971.3
Notional Amount of Derivatives		904.5	$ 1,052.0		799.2			$ 1,470.3
Interest Paid, Including Capitalized Interest, Operating and Investing Activities	$ 167.3	154.4	153.6
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 2020					400.0
Repayments of Principal in 2021					350.0
Repayments of Principal in 2023					2.0
Repayments of Principal Thereafter					2,105.6
Current Maturities of Long-term Debt		199.9			0.0
Proceeds from Issuance of Debt			600.0
Long-term Debt, Excluding Current Maturities		2,538.4			2,831.3
Interest Paid, Including Capitalized Interest, Operating and Investing Activities	161.4	148.7	$ 147.8
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
Notes Payable Due 2021 \| Parent Company
Stated Interest Rate of Debt			3.00%
Proceeds from Issuance of Debt			$ 350.0
Notes Issued in 2018 with a 2058 Maturity Date \| Junior Subordinated Debt
Stated Interest Rate of Debt							6.25%
Proceeds from Issuance of Debt	300.0
Long-term Debt, Excluding Current Maturities		0.0			300.0
Notes Issued in 2018 with a 2058 Maturity Date \| Junior Subordinated Debt \| Parent Company
Stated Interest Rate of Debt							6.25%
Proceeds from Issuance of Debt	$ 300.0
Long-term Debt, Excluding Current Maturities		0.0			300.0
Notes Issued in 1998 with 2018 Maturity Date \| Parent Company
Current Maturities of Long-term Debt					200.0
Stated Interest Rate of Debt							7.00%
Notes Issued in 2009 \| Parent Company
Current Maturities of Long-term Debt			$ 350.0
Stated Interest Rate of Debt			7.125%
Notes Issued in 1998 with 2038 Maturity Date \| Junior Subordinated Debt
Stated Interest Rate of Debt						7.405%	7.405%
Long-term Debt, Excluding Current Maturities		226.5			226.5
Proceeds from Issuance of Senior Long-term Debt				$ 300.0
Liquidation value per capital security	$ 1,000
Notes Issued in 1998 with 2038 Maturity Date \| Junior Subordinated Debt \| Parent Company
Stated Interest Rate of Debt						7.405%	7.405%
Long-term Debt, Excluding Current Maturities		226.5			226.5
Proceeds from Issuance of Senior Long-term Debt				$ 300.0
Liquidation value per capital security	$ 1,000
Notes Payable due 2042 \| Parent Company
Stated Interest Rate of Debt			5.75%
Proceeds from Issuance of Debt			$ 250.0
Notes Issued in 2012 \| Parent Company
Stated Interest Rate of Debt			5.75%
Long-term Debt, Excluding Current Maturities			$ 250.0
Receive Fixed/Pay Variable \| Interest Rate Swaps
Notional Amount of Derivatives		250.0			250.0
Receive Fixed/Pay Variable \| Interest Rate Swaps \| Parent Company
Notional Amount of Derivatives		$ 250.0			$ 250.0