Summary of Pension Plan Assets by Category Based on Hierarchy of Fair Value Measurements (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	$ 260.9	$ 248.5
Percent of total assets	100.00%	100.00%
Level 1 [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	$ 10.6	$ 23.1
Level 2 [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	250.3	225.4
Emerging Markets Bonds [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	$ 10.6	$ 9.1
Percent of total assets	4.00%	4.00%
Emerging Markets Bonds [Member] | Level 1 [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	$ 10.6	$ 9.1
Cash and Cash Equivalents [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	0.5	$ 13.2
Percent of total assets		5.00%
Cash and Cash Equivalents [Member] | Level 2 [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	0.5	$ 13.2
Emerging Markets Equity Fund [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets		$ 14.0
Percent of total assets		6.00%
Emerging Markets Equity Fund [Member] | Level 1 [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets		$ 14.0
Equity Securities [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	$ 132.4	$ 122.7
Percent of total assets	51.00%	50.00%
Equity Securities [Member] | Level 1 [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets		$ 14.0
Equity Securities [Member] | Level 2 [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	$ 132.4	108.7
Equity Securities [Member] | Global Large-Cap [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	$ 99.9
Percent of total assets	38.00%
Equity Securities [Member] | Global Large-Cap [Member] | Level 2 [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	$ 99.9
Equity Securities [Member] | U.S. Large-Cap [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets		$ 92.2
Percent of total assets		37.00%
Equity Securities [Member] | U.S. Large-Cap [Member] | Level 2 [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets		$ 92.2
Equity Securities [Member] | U.S. Small and Mid-Cap [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	$ 16.0	$ 16.5
Percent of total assets	6.00%	7.00%
Equity Securities [Member] | U.S. Small and Mid-Cap [Member] | Level 2 [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	$ 16.0	$ 16.5
Equity Securities [Member] | Emerging Markets Bonds [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	$ 16.5
Percent of total assets	6.00%
Equity Securities [Member] | Emerging Markets Bonds [Member] | Level 2 [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	$ 16.5
Private Real Estate Fund [Member] | High Yield Bonds [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	$ 7.4	$ 6.7
Percent of total assets	3.00%	3.00%
Private Real Estate Fund [Member] | High Yield Bonds [Member] | Level 2 [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	$ 7.4	$ 6.7
Private Real Estate Fund [Member] | Convertible Securities [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	$ 7.3
Percent of total assets	3.00%
Private Real Estate Fund [Member] | Convertible Securities [Member] | Level 2 [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	$ 7.3
Fixed Income Securities [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	$ 107.6	$ 94.8
Percent of total assets	41.00%	38.00%
Fixed Income Securities [Member] | Level 1 [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	$ 10.6	$ 9.1
Fixed Income Securities [Member] | Level 2 [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	97.0	85.7
Fixed Income Securities [Member] | Common Collective Trust Funds [Member] | Intermediate Term Investment Grade U.S. Government [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	$ 69.8	$ 60.8
Percent of total assets	27.00%	24.00%
Fixed Income Securities [Member] | Common Collective Trust Funds [Member] | Intermediate Term Investment Grade U.S. Government [Member] | Level 2 [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	$ 69.8	$ 60.8
Fixed Income Securities [Member] | Common Collective Trust Funds [Member] | U.S. Treasury Inflation-Protected Securities (TIPs) [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	$ 12.5	$ 10.7
Percent of total assets	5.00%	4.00%
Fixed Income Securities [Member] | Common Collective Trust Funds [Member] | U.S. Treasury Inflation-Protected Securities (TIPs) [Member] | Level 2 [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	$ 12.5	$ 10.7
Fixed Income Securities [Member] | Common Collective Trust Funds [Member] | Convertible Securities [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets		$ 7.5
Percent of total assets		3.00%
Fixed Income Securities [Member] | Common Collective Trust Funds [Member] | Convertible Securities [Member] | Level 2 [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets		$ 7.5
Total Other Investment [Member] | Private Real Estate Fund [Member] | Common/Collective Trust Funds - Convertible Securities [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	$ 20.4	$ 17.8
Percent of total assets	8.00%	7.00%
Total Other Investment [Member] | Private Real Estate Fund [Member] | Common/Collective Trust Funds - Convertible Securities [Member] | Level 2 [Member]
Schedule Of Pension And Other Retirement Benefits Changes In Benefit Obligation And Fair Value Of Plan Assets [Line Items]
Total assets	$ 20.4	$ 17.8