Consolidated Statements of Stockholders' Equity - USD ($) $ in Millions	Class A Common Stock	Class B Common Stock	Additional Paid-In Capital	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment [Member]	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Cumulative Effect, Period of Adoption, Adjustment [Member]	Total
Balance at the beginning 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
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	$ (0.7)	440.7	(121.1)	$ (0.7)	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
Increase (Decrease) in Stockholders' Equity
Change in accounting principle								513.9
Net income					114.3			114.3
Other comprehensive income						30.8		30.8
Comprehensive income								145.1
Shares of Class B common stock converted to Class A common stock (in shares)	135,000	(135,000)
Shares of Class A common stock issued upon the exercise of stock options			0.4					0.4
Shares of Class A common stock issued upon the exercise of stock options (in shares)	4,666
Stock-based compensation			12.7					12.7
Stock repurchase					(28.9)			(28.9)
Stock repurchase (in shares)	(331,531)
Net change in restricted stock units			1.7		(7.8)			(6.1)
Net change in restricted and performance stock units (in shares)	83,961
Common stock dividends					(31.4)			(31.4)
Balance at the end of the period at Dec. 31, 2020	$ 2.8	$ 0.6	$ 606.3		$ 560.1	$ (100.0)		1,069.8
Balance (in shares) at Dec. 31, 2020	27,478,512	6,144,290
Increase (Decrease) in Stockholders' Equity
Change in accounting principle								$ 560.1