Schedule II - (Parent Only) Schedule II - Condensed Cash Flows (Parent Only) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net Cash Provided by (Used in) Operating Activities		$ 530.4	$ 108.6	$ 229.9
Sales of:
Fixed maturity securities available for sale		2,923.1	2,963.5	2,380.8
Equity securities available for sale		97.5	223.3	181.9
Property, buildings and equipment	[1]	26.2	0.0	0.0
Subsidiary	[2]	0.0	873.9	49.9
Purchases of:
Fixed maturity securities available for sale		(3,547.2)	(4,260.0)	(2,747.4)
Equity securities available for sale		(24.4)	(200.5)	(185.0)
Other invested assets		(46.5)	(98.5)	(29.3)
Property and equipment and other		(62.1)	(85.2)	(114.9)
Change in short-term investments		(53.9)	273.2	(196.7)
Net cash (used in) provided by investing activities		(541.2)	725.6	264.3
Financing activities
Issuance of debt		0.0	249.6	0.0
Repayment of debt, including extinguishment		0.0	(373.0)	0.0
Acquisition of common stock		(388.9)	(863.1)	(292.9)
Dividends paid		(119.0)	(125.3)	(94.2)
Withholding on stock based compensation		19.5	26.0	24.7
Net cash (used in) provided by financing activities		(26.7)	(1,080.3)	(462.4)
Change in cash and cash equivalents		(35.2)	(256.3)	(30.3)
Cash and cash equivalents at beginning of period		1,032.0	1,288.3	1,318.6
Cash and cash equivalents at end of period		996.8	1,032.0	1,288.3
Parent Company
Operating activities
Net Cash Provided by (Used in) Operating Activities		177.1	981.1	636.9
Sales of:
Fixed maturity securities available for sale		589.8	441.0	442.8
Equity securities available for sale		9.7	10.2	32.3
Other invested assets		3.6	0.2	0.4
Property, buildings and equipment		26.2	0.0	0.0
Subsidiary		0.0	13.3	0.0
Maturities, calls, prepayments, and scheduled redemption of:
Fixed maturity securities available for sale		47.4	17.1	20.2
Purchases of:
Fixed maturity securities available for sale		(538.2)	(480.2)	(461.7)
Equity securities available for sale		(3.9)	(25.6)	(13.3)
Other invested assets		(24.1)	(3.7)	(2.7)
Property and equipment and other		(23.5)	(26.3)	(47.5)
Capital contributed to subsidiaries		(186.6)	(86.5)	(439.5)
Return of capital contributions from subsidiaries		41.9	3.6	172.4
Change in short-term investments		248.8	154.9	5.0
Net cash (used in) provided by investing activities		191.1	18.0	(291.6)
Financing activities
Issuance of debt		0.0	249.6	0.0
Repayment of debt, including extinguishment		0.0	(373.0)	0.0
Change in tax benefit from share-based payment arrangements		0.0	5.6	(4.0)
Acquisition of common stock		(388.9)	(863.1)	(292.9)
Dividends paid		(118.9)	(125.3)	(94.2)
Withholding on stock based compensation		10.8	13.1	12.4
Net cash (used in) provided by financing activities		(497.0)	(1,093.1)	(378.7)
Cash included in held for sale assets		0.0	4.7	(4.7)
Change in cash and cash equivalents		(128.8)	(89.3)	(38.1)
Cash and cash equivalents at beginning of period		264.8	354.1	392.2
Cash and cash equivalents at end of period		$ 136.0	$ 264.8	$ 354.1

[1] 2017 represents total cash received from the sale of a building that had been the headquarters of our Employee Benefits business.

[2] 2017 primarily includes the acquisition of Green Tree Insurance Agency, Inc. 2016 includes the sale of our Employee Benefits segment mainly through reinsurance transactions and supplemental and small group self-funded business; the acquisition of American Title and the purchase of renewal rights to the National Flood Insurance block of business of Nationwide Mutual Insurance Company and other immaterial subsidiaries. 2015 includes the sale of American Reliable Insurance Co. and certain assets related to our vehicle title administration services business and supplemental and small group self-funded businesses; the acquisition of certain other entities.