Pension and Postretirement Benefit Plans - Company's Share of Postretirement 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
Postretirement
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value		0.3
Postretirement | Fixed maturities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value		0.1
Postretirement | Fixed maturities: | Corporate securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value		0.1
Postretirement | Money Market Funds [Member] | Money Market Funds [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value		0.2
Quoted prices in active markets for identical assets (Level 1) | Postretirement
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value		0.2
Quoted prices in active markets for identical assets (Level 1) | Postretirement | Fixed maturities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value		0.0
Quoted prices in active markets for identical assets (Level 1) | Postretirement | Fixed maturities: | Corporate securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value		0.0
Quoted prices in active markets for identical assets (Level 1) | Postretirement | Money Market Funds [Member] | Money Market Funds [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value		0.2
Significant other observable inputs (Level 2) | Postretirement
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value		0.1
Significant other observable inputs (Level 2) | Postretirement | Fixed maturities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value		0.1
Significant other observable inputs (Level 2) | Postretirement | Fixed maturities: | Corporate securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value		0.1
Significant other observable inputs (Level 2) | Postretirement | Money Market Funds [Member] | Money Market Funds [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value		$ 0.0