Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Millions	Total	Common Stock Issued	Additional Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive (Loss) Income
Balance (in shares) at Dec. 31, 2016		30,544,494
Balance at Dec. 31, 2016	$ 508.3	$ 3.1	$ 59.2	$ 585.3	$ (139.3)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	34.5			34.5
Other comprehensive income (loss), net of tax	49.9				49.9
Dividends declared	(51.9)			(51.9)
Restricted stock issuances, net (in shares)		186,867
Restricted stock issuances, net	0.0
Stock-based employee compensation expense	6.9		6.9
Stock issued to directors as compensation (in shares)		5,798
Stock issued to directors as compensation	0.2		0.2
Purchases and cancellation of common stock (in shares)		(25,860)
Purchases and cancellation of common stock	(1.2)			(1.2)
Balance (in shares) at Dec. 31, 2017		30,711,299
Balance at Dec. 31, 2017	546.7	$ 3.1	66.3	566.7	(89.4)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	94.5			94.5
Other comprehensive income (loss), net of tax	(30.2)				(30.2)
Dividends declared	(53.2)			(53.2)
Restricted stock issuances, net (in shares)		130,617
Restricted stock issuances, net	0.0
Stock-based employee compensation expense	4.6		4.6
Stock issued to directors as compensation (in shares)		4,723
Stock issued to directors as compensation	0.2		0.2
Purchases and cancellation of common stock (in shares)		(75,395)
Purchases and cancellation of common stock	(3.0)			(3.0)
Balance (in shares) at Dec. 31, 2018		30,771,244
Balance at Dec. 31, 2018	557.9	$ 3.1	71.1	608.2	(124.5)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	85.8			85.8
Other comprehensive income (loss), net of tax	1.9				1.9
Dividends declared	(54.4)			(54.4)
Restricted stock issuances, net (in shares)		147,113
Restricted stock issuances, net	0.0
Stock-based employee compensation expense	7.6		7.6
Stock issued to directors as compensation (in shares)		3,601
Stock issued to directors as compensation	0.1		0.1
Purchases and cancellation of common stock (in shares)		(25,297)
Purchases and cancellation of common stock	(0.9)			(0.9)
Balance (in shares) at Dec. 31, 2019		30,896,661
Balance at Dec. 31, 2019	$ 597.7	$ 3.1	$ 78.8	$ 638.4	$ (122.6)