Reserves (Narrative) (Details) - USD ($) $ in Millions	Jun. 30, 2018	May 31, 2018	Dec. 31, 2017	Jun. 30, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Goodwill	$ 2,369.2		$ 917.7
Global Housing and Global Lifestyle
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable development	29.3			$ 51.7
Global Housing
Liability for Claims and Claims Adjustment Expense [Line Items]
Goodwill	378.6		$ 386.7
Global Housing | Hurricane Matthew
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable development	2.2			5.2
Assurant Health
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable development	$ 0.7			$ 7.8
TWG Holdings Limited
Liability for Claims and Claims Adjustment Expense [Line Items]
Goodwill		$ 1,463.8