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]	Retained Earnings [Member] Cumulative Effect, Period of Adoption, Adjustment
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 Sep. 30, 2019		(50.3)	(6.8)
Balance at beginning of year at Dec. 31, 2018		$ (125.0)	$ (117.0)	$ (194.2)	$ 96.4	$ (712.7)
Balance at end of year at Sep. 30, 2019	$ (943.7)	(125.8)	(117.5)	(205.2)	24.2	(754.4)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares		0.8		4.0
Shares acquired on stock award exercises and vested restricted shares			$ (0.5)
Stock awards granted				7.0
Change in net unrealized holding (losses) gains, net of tax	70.3				70.3
Change in unrecognized benefit plan obligations, net of tax and reclassification adjustments	1.9				1.9
Net income (loss)	54.7					54.7
Cash dividends paid		$ (13.0)
Cumulative effect of change in accounting to establish an allowance for expected credit losses | Accounting Standards Update 2016-13							$ 0.0
Balance at beginning of year (shares) at Jun. 30, 2019		(50.3)	(6.8)
Balance at end of year (shares) at Sep. 30, 2019		(50.3)	(6.8)
Balance at beginning of year at Jun. 30, 2019		$ (125.7)	$ (117.5)	(202.5)	41.7	(747.2)
Balance at end of year at Sep. 30, 2019	(943.7)	(125.8)	$ (117.5)	(205.2)	24.2	(754.4)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares		0.1		0.7
Stock awards granted				2.0
Change in net unrealized holding (losses) gains, net of tax	16.9				16.9
Change in unrecognized benefit plan obligations, net of tax and reclassification adjustments	0.6				0.6
Net income (loss)	$ 11.5					11.5
Cash dividends paid		$ (4.3)
Document Fiscal Year Focus	2020
Balance at beginning of year (shares) at Dec. 31, 2019		(50.4)	(6.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares (shares)		0.3
Balance at end of year (shares) at Sep. 30, 2020		(50.7)	(6.9)
Balance at beginning of year at Dec. 31, 2019	$ (959.9)	$ (125.9)	$ (117.5)	(206.7)	37.9	(782.7)
Balance at end of year at Sep. 30, 2020	(944.1)	(126.7)	(118.4)	(212.1)	(23.3)	(700.4)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares		0.8		2.0
Shares acquired on stock award exercises and vested restricted shares			$ (0.9)
Stock awards granted				3.4
Change in net unrealized holding (losses) gains, net of tax	56.3				56.3
Change in unrecognized benefit plan obligations, net of tax and reclassification adjustments	4.9				4.9
Net income (loss)	(68.7)					(68.7)
Cash dividends paid		$ (13.1)
Cumulative effect of change in accounting to establish an allowance for expected credit losses | Accounting Standards Update 2016-13							$ (0.5)
Balance at beginning of year (shares) at Jun. 30, 2020		(50.7)	(6.9)
Balance at end of year (shares) at Sep. 30, 2020		(50.7)	(6.9)
Balance at beginning of year at Jun. 30, 2020		$ (126.7)	$ (118.4)	(211.1)	(17.9)	(693.2)
Balance at end of year at Sep. 30, 2020	(944.1)	(126.7)	$ (118.4)	(212.1)	(23.3)	(700.4)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares		0.0		(0.2)
Stock awards granted				$ 1.2
Change in net unrealized holding (losses) gains, net of tax	3.7				3.7
Change in unrecognized benefit plan obligations, net of tax and reclassification adjustments	1.7				$ 1.7
Net income (loss)	$ 11.6					$ 11.6
Cash dividends paid		$ (4.4)