Condensed Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Millions	Total	Common Stock Issued	Additional Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Beginning Balance at Dec. 31, 2017	$ 546.7	$ 3.1	$ 66.3	$ 566.7	$ (89.4)
Beginning Balance (in shares) at Dec. 31, 2017		30,711,299
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	87.3			87.3
Other comprehensive (loss), net of tax	(24.8)				(24.8)
Dividends declared	(39.7)			(39.7)
Restricted stock issuances, net (in shares)		122,817
Stock-based employee compensation expense	3.5		3.5
Stock issued to directors as compensation	0.2		0.2
Stock issued to directors as compensation (in shares)		3,615
Purchases and retirement of common stock	(2.8)			(2.8)
Purchases and retirement of common stock (in shares)		(69,758)
Ending Balance at Sep. 30, 2018	568.7	$ 3.1	70.0	609.8	(114.2)
Ending Balance (in shares) at Sep. 30, 2018		30,767,973
Beginning Balance at Jun. 30, 2018	542.5	$ 3.1	68.8	582.2	(111.6)
Beginning Balance (in shares) at Jun. 30, 2018		30,754,759
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	41.0			41.0
Other comprehensive (loss), net of tax	(2.6)				(2.6)
Dividends declared	(13.3)			(13.3)
Restricted stock issuances, net (in shares)		14,663
Stock-based employee compensation expense	1.1		1.1
Stock issued to directors as compensation (in shares)		972
Purchases and retirement of common stock	(0.1)			(0.1)
Purchases and retirement of common stock (in shares)		(2,421)
Ending Balance at Sep. 30, 2018	568.7	$ 3.1	70.0	609.8	(114.2)
Ending Balance (in shares) at Sep. 30, 2018		30,767,973
Beginning Balance at Dec. 31, 2018	557.9	$ 3.1	71.1	608.2	(124.5)
Beginning Balance (in shares) at Dec. 31, 2018		30,771,244
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	65.6			65.6
Other comprehensive (loss), net of tax	(10.6)				(10.6)
Dividends declared	(40.8)			(40.8)
Restricted stock issuances, net (in shares)		147,019
Stock-based employee compensation expense	3.9		3.9
Stock issued to directors as compensation	0.1		0.1
Stock issued to directors as compensation (in shares)		3,034
Purchases and retirement of common stock	(1.0)			(1.0)
Purchases and retirement of common stock (in shares)		(25,297)
Ending Balance at Sep. 30, 2019	574.8	$ 3.1	75.1	631.7	(135.1)
Ending Balance (in shares) at Sep. 30, 2019		30,896,000
Beginning Balance at Jun. 30, 2019	572.6	$ 3.1	73.6	617.6	(121.7)
Beginning Balance (in shares) at Jun. 30, 2019		30,894,598
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	27.7			27.7
Other comprehensive (loss), net of tax	(13.4)				(13.4)
Dividends declared	(13.6)			(13.6)
Restricted stock issuances, net (in shares)		914
Stock-based employee compensation expense	1.5		1.5
Stock issued to directors as compensation	0.0		0.0
Stock issued to directors as compensation (in shares)		640
Purchases and retirement of common stock	0.0			0.0
Purchases and retirement of common stock (in shares)		(152)
Ending Balance at Sep. 30, 2019	$ 574.8	$ 3.1	$ 75.1	$ 631.7	$ (135.1)
Ending Balance (in shares) at Sep. 30, 2019		30,896,000