Schedule II - Condensed Balance Sheet (Parent Only) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investments:
Fixed maturity securities available for sale, at fair value (amortized cost – $375.4 in 2017 and $428.7 in 2016)	$ 9,662.6	$ 9,572.1
Equity securities available for sale, at fair value (amortized cost – $14.6 in 2017 and $20.3 in 2016)	368.0	421.4
Short-term investments	284.1	227.7
Other investments	568.6	633.8
Total investments	11,553.5	11,479.0
Cash and cash equivalents	996.8	1,032.0	$ 1,288.3	$ 1,318.6
Income tax receivable	126.3	20.5
Accrued investment income	105.4	110.1
Property and equipment, at cost less accumulated depreciation	347.6	343.6
Other assets	387.1	359.7
Total assets	31,843.0	29,709.1
Liabilities
Accounts payable and other liabilities	2,046.3	1,985.7
Debt	1,068.2	1,067.0
Total liabilities	27,561.5	25,611.0
Commitments and Contingencies
Stockholders’ equity
Common stock, par value $0.01 per share, 800,000,000 shares authorized, 52,417,812 and 55,941,480 shares outstanding at December 31, 2017 and 2016, respectively	1.5	1.5
Additional paid-in capital	3,197.9	3,175.9
Retained earnings	5,697.3	5,296.7
Accumulated other comprehensive income	234.0	94.6
Treasury stock, at cost; 97,974,792 and 94,041,583 shares at December 31, 2017 and 2016, respectively	(4,860.1)	(4,470.6)
Total Assurant, Inc. stockholders’ equity	4,270.6	4,098.1
Total liabilities and equity	31,843.0	29,709.1
Parent Company
Investments:
Equity investment in subsidiaries	4,674.3	4,147.6
Fixed maturity securities available for sale, at fair value (amortized cost – $375.4 in 2017 and $428.7 in 2016)	382.2	429.2
Equity securities available for sale, at fair value (amortized cost – $14.6 in 2017 and $20.3 in 2016)	16.1	21.1
Short-term investments	14.2	220.0
Other investments	115.9	92.4
Total investments	5,202.7	4,910.3
Cash and cash equivalents	136.0	264.8	$ 354.1	$ 392.2
Receivable from subsidiaries, net	58.4	42.5
Income tax receivable	26.8	0.0
Accrued investment income	3.8	4.9
Property and equipment, at cost less accumulated depreciation	118.1	134.3
Other assets	39.7	27.5
Total assets	5,585.5	5,384.3
Liabilities
Accounts payable and other liabilities	246.7	198.9
Income tax payable	0.0	20.3
Debt	1,068.2	1,067.0
Total liabilities	1,314.9	1,286.2
Commitments and Contingencies
Stockholders’ equity
Common stock, par value $0.01 per share, 800,000,000 shares authorized, 52,417,812 and 55,941,480 shares outstanding at December 31, 2017 and 2016, respectively	1.5	1.5
Additional paid-in capital	3,197.9	3,175.9
Retained earnings	5,697.3	5,296.7
Accumulated other comprehensive income	234.0	94.6
Treasury stock, at cost; 97,974,792 and 94,041,583 shares at December 31, 2017 and 2016, respectively	(4,860.1)	(4,470.6)
Total Assurant, Inc. stockholders’ equity	4,270.6	4,098.1
Total liabilities and equity	$ 5,585.5	$ 5,384.3