Consolidated Statements of Stockholders' Equity - USD ($) $ in Millions	Class A Common Stock	Class B Common Stock	Additional Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Total
Balance at the beginning of the period at Dec. 31, 2016	$ 2.8	$ 0.6	$ 535.2	$ 348.5	$ (150.8)	$ 736.3
Balance (in shares) at Dec. 31, 2016	27,831,013	6,379,290
Increase (Decrease) in Stockholders' Equity
Change in accounting principle				(0.5)		(0.5)
Net income				73.1		73.1
Other comprehensive income					51.7	51.7
Comprehensive income						124.8
Shares of Class A common stock issued upon the exercise of stock options			1.7			1.7
Shares of Class A common stock issued upon the exercise of stock options (in shares)	31,377
Stock-based compensation			13.9			13.9
Stock repurchase				(18.2)		(18.2)
Stock repurchase (in shares)	(277,886)
Issuance of net shares of restricted Class A common stock				(2.4)		(2.4)
Issuance of net shares of restricted Class A common stock (in shares)	87,443
Net change in restricted stock units			1.0	(1.7)		(0.7)
Net change in restricted and performance stock units (in shares)	52,245
Common stock dividends				(25.9)		(25.9)
Balance at the end of the period at Dec. 31, 2017	$ 2.8	$ 0.6	551.8	372.9	(99.1)	829.0
Balance (in shares) at Dec. 31, 2017	27,724,192	6,379,290
Increase (Decrease) in Stockholders' Equity
Change in accounting principle				(0.7)		(0.7)
Net income				128.0		128.0
Other comprehensive income					(22.0)	(22.0)
Comprehensive income						106.0
Shares of Class B common stock converted to Class A common stock (in shares)	50,000	(50,000)
Shares of Class A common stock issued upon the exercise of stock options			2.5			2.5
Shares of Class A common stock issued upon the exercise of stock options (in shares)	45,939
Stock-based compensation			13.8			13.8
Stock repurchase				(26.0)		(26.0)
Stock repurchase (in shares)	(340,106)
Issuance of net shares of restricted Class A common stock				(3.1)		(3.1)
Issuance of net shares of restricted Class A common stock (in shares)	115,120
Net change in restricted stock units			0.2	(2.1)		(1.9)
Net change in restricted and performance stock units (in shares)	51,320
Common stock dividends				(28.3)		(28.3)
Balance at the end of the period at Dec. 31, 2018	$ 2.8	$ 0.6	568.3	440.7	(121.1)	891.3
Balance (in shares) at Dec. 31, 2018	27,646,465	6,329,290
Increase (Decrease) in Stockholders' Equity
Net income				131.5		131.5
Other comprehensive income					(9.7)	(9.7)
Comprehensive income						121.8
Shares of Class B common stock converted to Class A common stock (in shares)	50,000	(50,000)
Shares of Class A common stock issued upon the exercise of stock options			2.1			2.1
Shares of Class A common stock issued upon the exercise of stock options (in shares)	38,288
Stock-based compensation			17.8			17.8
Stock repurchase				(19.5)		(19.5)
Stock repurchase (in shares)	(227,620)
Net change in restricted stock units			3.3	(7.4)		(4.1)
Net change in restricted and performance stock units (in shares)	79,283
Common stock dividends				(31.4)		(31.4)
Balance at the end of the period at Dec. 31, 2019	$ 2.8	$ 0.6	$ 591.5	$ 513.9	$ (130.8)	$ 978.0
Balance (in shares) at Dec. 31, 2019	27,586,416	6,279,290