Pension and Postretirement Benefit Plans - Company's Share of Pension Plans' Available-for-Sale Securities (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Document Period End Date	Dec. 31, 2017
Available-for-sale Securities	$ 2,614.1	$ 2,537.2
Pension
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	231.9	200.9
Pension | Short-term Investments [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities		2.0
Pension | Fixed maturities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	135.2	113.0
Pension | Fixed maturities: | U.S. treasury securities and obligations of U.S. government agencies
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	70.1	60.4
Pension | Fixed maturities: | Corporate securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	60.7	48.0
Pension | Fixed maturities: | U.S. government agencies mortgage-backed securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	4.4	4.6
Pension | Equity Securities [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	96.7	85.9
Pension | Equity Securities [Member] | Large-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	71.6	59.6
Pension | Equity Securities [Member] | Small-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities		26.3
Pension | Equity Securities [Member] | Mutual and exchange traded funds [Domain] [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	25.1
Pension | Quoted prices in active markets for identical assets (Level 1)
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	96.7	85.9
Pension | Quoted prices in active markets for identical assets (Level 1) | Fixed maturities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	0.0	0.0
Pension | Quoted prices in active markets for identical assets (Level 1) | Fixed maturities: | U.S. treasury securities and obligations of U.S. government agencies
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	0.0	0.0
Pension | Quoted prices in active markets for identical assets (Level 1) | Fixed maturities: | Corporate securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	0.0	0.0
Pension | Quoted prices in active markets for identical assets (Level 1) | Fixed maturities: | U.S. government agencies mortgage-backed securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	0.0	0.0
Pension | Quoted prices in active markets for identical assets (Level 1) | Equity Securities [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	96.7	85.9
Pension | Quoted prices in active markets for identical assets (Level 1) | Equity Securities [Member] | Large-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	71.6	59.6
Pension | Quoted prices in active markets for identical assets (Level 1) | Equity Securities [Member] | Small-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities		26.3
Pension | Quoted prices in active markets for identical assets (Level 1) | Equity Securities [Member] | Mutual and exchange traded funds [Domain] [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	25.1
Pension | Quoted prices in active markets for identical assets (Level 1) | Short-term Investments [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities		0.0
Pension | Significant other observable inputs (Level 2)
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	135.2	115.0
Pension | Significant other observable inputs (Level 2) | Fixed maturities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	135.2	113.0
Pension | Significant other observable inputs (Level 2) | Fixed maturities: | U.S. treasury securities and obligations of U.S. government agencies
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	70.1	60.4
Pension | Significant other observable inputs (Level 2) | Fixed maturities: | Corporate securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	60.7	48.0
Pension | Significant other observable inputs (Level 2) | Fixed maturities: | U.S. government agencies mortgage-backed securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	4.4	4.6
Pension | Significant other observable inputs (Level 2) | Equity Securities [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	0.0	0.0
Pension | Significant other observable inputs (Level 2) | Equity Securities [Member] | Large-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	0.0	0.0
Pension | Significant other observable inputs (Level 2) | Equity Securities [Member] | Small-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities		0.0
Pension | Significant other observable inputs (Level 2) | Equity Securities [Member] | Mutual and exchange traded funds [Domain] [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	$ 0.0
Pension | Significant other observable inputs (Level 2) | Short-term Investments [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities		$ 2.0