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, 2012		47.3	6.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares (shares)		0.2
Balance at end of year (shares) at Dec. 31, 2013		47.5	6.8
Balance at beginning of year at Dec. 31, 2012		$ 118.1	$ (115.8)	$ 131.6	$ 84.2	$ 519.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares		0.7		3.4
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				2.5
Change in unrealized holding gains (losses) on investments, net of tax	$ (39.4)				(39.4)
Amortization of gain on derivative used in cash flow hedge					(0.1)
Change in unrecognized benefit plan obligations, net of tax and reclassification adjustments	36.1				36.1
Net income	60.8					60.8
Cash dividends paid (affiliates $10.4, $10.2 and $10.1, respectively)	(16.1)
Balance at end of year at Dec. 31, 2013	785.0	$ 118.8	$ (115.9)	137.5	80.8	563.8
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
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
Amortization of gain on derivative used in cash flow hedge					0.0
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, $10.2 and $10.1, 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)
Amortization of gain on derivative used in cash flow hedge					0.0
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, $10.2 and $10.1, respectively)	(16.5)
Balance at end of year at Dec. 31, 2015	$ 884.6	$ 120.4	$ (116.3)	$ 153.5	$ 37.6	$ 689.4