Pension and Postretirement Benefit Plans - Company's Share of Pension Plans' Available-for-Sale Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	$ 2,396.4	$ 2,282.6
Pension
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	196.2	199.7
Pension | International instruments
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	18.6	18.7
Pension | Fixed maturities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	96.3	105.5
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]
Asset Available-for-sale, at fair value	51.0	56.4
Pension | Fixed maturities: | Corporate securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	38.9	39.7
Pension | Fixed maturities: | U.S. government agencies mortgage-backed securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	6.4	9.4
Pension | Equity securities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	81.3	75.5
Pension | Equity securities: | Large-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	59.0	53.7
Pension | Equity securities: | Small-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	22.3	21.8
Pension | Quoted prices in active markets for identical assets (Level 1)
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	81.3	75.5
Pension | Quoted prices in active markets for identical assets (Level 1) | International instruments
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	0.0	0.0
Pension | Quoted prices in active markets for identical assets (Level 1) | Fixed maturities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	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]
Asset Available-for-sale, at fair value	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]
Asset Available-for-sale, at fair value	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]
Asset Available-for-sale, at fair value	0.0	0.0
Pension | Quoted prices in active markets for identical assets (Level 1) | Equity securities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	81.3	75.5
Pension | Quoted prices in active markets for identical assets (Level 1) | Equity securities: | Large-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	59.0	53.7
Pension | Quoted prices in active markets for identical assets (Level 1) | Equity securities: | Small-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	22.3	21.8
Pension | Significant other observable inputs (Level 2)
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	114.9	124.2
Pension | Significant other observable inputs (Level 2) | International instruments
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	18.6	18.7
Pension | Significant other observable inputs (Level 2) | Fixed maturities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	96.3	105.5
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]
Asset Available-for-sale, at fair value	51.0	56.4
Pension | Significant other observable inputs (Level 2) | Fixed maturities: | Corporate securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	38.9	39.7
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]
Asset Available-for-sale, at fair value	6.4	9.4
Pension | Significant other observable inputs (Level 2) | Equity securities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	0.0	0.0
Pension | Significant other observable inputs (Level 2) | Equity securities: | Large-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	0.0	0.0
Pension | Significant other observable inputs (Level 2) | Equity securities: | Small-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	$ 0.0	$ 0.0