Condensed Consolidated Statement of Stockholders' Equity Statement - USD ($) shares in Millions, $ in Millions	Total	Common Stock [Member]	Treasury Stock [Member]	Additional Paid-in Capital [Member]	AOCI Attributable to Parent [Member]	Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative Effect of change in accounting for equity securities and other invested assets and reclassification of stranded tax effects as of January 1, 2018	$ (63.9)
Balance at beginning of year (shares) at Dec. 31, 2017		(49.2)	(6.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares (shares)		0.5
Balance at end of year (shares) at Jun. 30, 2018		(49.7)	(6.8)
Balance at beginning of year at Dec. 31, 2017		$ 123.0	$ (116.8)	$ 171.8	$ 3.8	$ 653.2
Balance at end of year at Jun. 30, 2018	808.5	124.1	(116.9)	184.8	(95.8)	712.3
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares		1.1		8.1
Shares acquired on stock award exercises and vested restricted shares			$ (0.1)
Stock awards granted				4.9
Change in net unrealized holding (losses) gains, net of tax	(38.4)				(38.4)
Change in unrecognized benefit plan obligations, net of tax and reclassification adjustments	2.7				2.7
Net income (loss)	3.9					3.9
Cash dividends paid		$ (8.7)
Balance at beginning of year (shares) at Mar. 31, 2018		(49.5)	(6.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares (shares)		0.2
Balance at end of year (shares) at Jun. 30, 2018		(49.7)	(6.8)
Balance at beginning of year at Mar. 31, 2018		$ 123.8	$ (116.9)	180.3	(88.1)	710.7
Balance at end of year at Jun. 30, 2018	808.5	124.1	(116.9)	184.8	(95.8)	712.3
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares		0.3		1.9
Shares acquired on stock award exercises and vested restricted shares			$ 0.0
Stock awards granted				2.6
Change in net unrealized holding (losses) gains, net of tax	(9.1)				(9.1)
Change in unrecognized benefit plan obligations, net of tax and reclassification adjustments	1.4				1.4
Net income (loss)	$ 6.0					6.0
Cash dividends paid		$ (4.4)
Cumulative Effect of change in accounting for equity securities and other invested assets and reclassification of stranded tax effects as of January 1, 2018					(63.9)	63.9
Document Fiscal Year Focus	2019
Balance at beginning of year (shares) at Dec. 31, 2018		(50.0)	(6.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares (shares)		0.3
Balance at end of year (shares) at Jun. 30, 2019		(50.3)	(6.8)
Balance at beginning of year at Dec. 31, 2018	$ 818.5	$ 125.0	$ (117.0)	194.2	(96.4)	712.7
Balance at end of year at Jun. 30, 2019	916.2	125.7	(117.5)	202.5	(41.7)	747.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares		0.7		3.3
Shares acquired on stock award exercises and vested restricted shares			$ (0.5)
Stock awards granted				5.0
Change in net unrealized holding (losses) gains, net of tax	53.4				53.4
Change in unrecognized benefit plan obligations, net of tax and reclassification adjustments	1.3				1.3
Net income (loss)	43.2					43.2
Cash dividends paid		$ (8.7)
Balance at beginning of year (shares) at Mar. 31, 2019		(50.1)	(6.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares (shares)		0.2
Balance at end of year (shares) at Jun. 30, 2019		(50.3)	(6.8)
Balance at beginning of year at Mar. 31, 2019		$ 125.3	$ (117.5)	198.4	(68.6)	757.7
Balance at end of year at Jun. 30, 2019	916.2	125.7	(117.5)	202.5	(41.7)	747.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares		0.4		2.6
Shares acquired on stock award exercises and vested restricted shares			$ 0.0
Stock awards granted				$ 1.5
Change in net unrealized holding (losses) gains, net of tax	26.2				26.2
Change in unrecognized benefit plan obligations, net of tax and reclassification adjustments	0.7				0.7
Net income (loss)	$ (6.2)					(6.2)
Cash dividends paid		$ (4.3)
Cumulative Effect of change in accounting for equity securities and other invested assets and reclassification of stranded tax effects as of January 1, 2018					$ 0.0	$ 0.0