Investments (Proceeds From Sales Of Available-For-Sale Securities And The Gross Realized Gains And Gross Realized Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sales	$ 3,583.6
Proceeds from sales, before ASU 2016-01		$ 3,018.2	$ 4,610.7
Gross realized gains	22.3
Gross realized gains, before ASU 2016-01		43.5	209.5
Gross realized losses	$ 60.0
Gross realized losses, before ASU 2016-01		$ 12.2	65.2
Disposal group, disposed of by sale, not discontinued operations | Assurant Employee Benefits
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross realized gains, before ASU 2016-01			150.7
Gross realized losses, before ASU 2016-01			$ 16.4