Consolidated Statements of Stockholders' Equity - USD ($) shares in Millions, $ in Millions	Total	Common shares:	Treasury shares:	Additional paid-in capital:	Accumulated other comprehensive income:	Retained earnings:
Balance at beginning of year (shares) at Dec. 31, 2013		47.5	6.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares (shares)		0.2
Balance at end of year (shares) at Dec. 31, 2014		47.7	6.8
Balance at beginning of year at Dec. 31, 2013		$ 118.8	$ (115.9)	$ 137.5	$ 80.8	$ 563.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares		0.5		2.9
Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation				0.0
Shares acquired on stock option exercises and vested restricted shares			(0.1)
Stock options granted				2.8
Change in unrealized holding gains (losses) on investments, net of tax	$ 25.4				25.4
Change in unrecognized benefit plan obligations, net of tax and reclassification adjustments	(34.5)				(34.5)
Net income	107.4					107.4
Cash dividends paid (affiliates $10.4, $10.4 and $10.2, respectively)	(16.5)
Balance at end of year at Dec. 31, 2014	872.9	$ 119.3	$ (116.0)	143.2	71.7	654.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares (shares)		0.4
Balance at end of year (shares) at Dec. 31, 2015		48.1	6.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares		$ 1.1		5.2
Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation				0.3
Shares acquired on stock option exercises and vested restricted shares			$ (0.3)
Stock options granted				4.8
Change in unrealized holding gains (losses) on investments, net of tax	(41.5)				(41.5)
Change in unrecognized benefit plan obligations, net of tax and reclassification adjustments	7.4				7.4
Net income	51.2					51.2
Cash dividends paid (affiliates $10.4, $10.4 and $10.2, respectively)	(16.5)
Balance at end of year at Dec. 31, 2015	884.6	$ 120.4	$ (116.3)	153.5	37.6	689.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares (shares)		0.5
Balance at end of year (shares) at Dec. 31, 2016		48.6	6.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares		$ 1.2		6.8
Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation				(3.0)
Shares acquired on stock option exercises and vested restricted shares			$ (0.2)
Stock options granted				2.6
Change in unrealized holding gains (losses) on investments, net of tax	(5.7)				(5.7)
Change in unrecognized benefit plan obligations, net of tax and reclassification adjustments	0.6				0.6
Net income	21.0					21.0
Cash dividends paid (affiliates $10.4, $10.4 and $10.2, respectively)	(16.6)
Balance at end of year at Dec. 31, 2016	$ 891.3	$ 121.6	$ (116.5)	$ 159.9	$ 32.5	$ 693.8