Statutory Financial Information - Additional Information (Detail) - USD ($) $ in Millions |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|
| Statutory Financial Information Disclosures [Abstract] | |||
| Consolidated statutory policyholders' surplus | $ 9,664.4 | $ 8,560.0 | |
| Statutory net income | 1,416.2 | $ 1,022.3 | $ 1,333.1 |
| Consolidated statutory policyholders' surplus, net admitted assets of insurance subsidiaries and affiliate that are required to meet minimum statutory surplus requirements in such entities' states of domicile | 830.1 | ||
| Aggregate cash dividends paid to the parent company by subsidiaries | 872.8 | ||
| Maximum aggregate dividend amount subsidiaries could pay without prior approval from regulatory authorities | $ 1,390.5 | ||