Consolidated Statement of Changes in Stockholders' Equity - USD ($) $ in Millions	Total	Common stock	Retained earnings	Accumulated other comprehensive income (loss)
Equity, beginning balance at Dec. 31, 2017	$ 1,427.6	$ 687.0	$ 752.6	$ (12.0)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	78.4		78.4
Reclassification of the income tax effects of the Tax Cut and Jobs Act from AOCI	0.0		3.1	(3.1)
Other comprehensive income (loss), net of tax expense	(19.9)			(19.9)
Common stock transactions:
During the three and six months ended June 30, 2019, there were 607 common shares purchased and retired, and 39,360 common shares purchased and retired, respectively. There were 896 common shares purchased and retired, and 22,596 common shares purchased and retired, during the same periods in 2018	(0.9)	(0.9)
During the three and six months ended June 30, 2019, there were 4,356,498 common shares issued, and 4,356,498 common shares issued, respectively. There were 11,389 common shares issued, and 11,389 common shares issued, during the same periods in 2018	0.0	0.0
During the three and six months ended June 30, 2019, there were 11,136 non-vested common shares issued, and 211,164 non-vested common shares issued, respectively. There were 204,991 non-vested common shares issued during the six months ended June 30, 2018	0.0	0.0
During the three and six months ended June 30, 2019, there were 16,032 non-vested common shares forfeited or canceled, and 21,417 non-vested common shares forfeited or canceled, respectively. There were 10,566 non-vested common shares forfeited or canceled, and 27,147 non-vested common shares forfeited or canceled, during the same periods in 2018	0.0	0.0
During the three and six months ended June 30, 2019, there were 41,704 stock options exercised, net of 16,104 shares tendered in payment of option price and income tax withholding amounts, and 98,686 stock options exercised, net of 35,108 shares tendered in payment of option price and income tax withholding amounts, respectively. There were 60,852 stock options exercised, net of 4,260 shares tendered in payment of option price and income tax withholding amounts, and 127,998 stock options exercised, net of	1.7	1.7
Stock-based compensation expense	2.9	2.9
Common cash dividend declared, $0.31 for the three months ended March 31 and June 30, 2019, respectively. There was $0.28 per share declared for the same periods in 2018	(31.5)		(31.5)
Equity, ending balance at Jun. 30, 2018	1,458.3	690.7	802.6	(35.0)
Equity, beginning balance at Mar. 31, 2018	1,432.4	688.0	776.7	(32.3)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	41.7		41.7
Other comprehensive income (loss), net of tax expense	(2.7)			(2.7)
Common stock transactions:
During the three and six months ended June 30, 2019, there were 607 common shares purchased and retired, and 39,360 common shares purchased and retired, respectively. There were 896 common shares purchased and retired, and 22,596 common shares purchased and retired, during the same periods in 2018	0.0	0.0
During the three and six months ended June 30, 2019, there were 4,356,498 common shares issued, and 4,356,498 common shares issued, respectively. There were 11,389 common shares issued, and 11,389 common shares issued, during the same periods in 2018	0.0	0.0
During the three and six months ended June 30, 2019, there were 16,032 non-vested common shares forfeited or canceled, and 21,417 non-vested common shares forfeited or canceled, respectively. There were 10,566 non-vested common shares forfeited or canceled, and 27,147 non-vested common shares forfeited or canceled, during the same periods in 2018	0.0	0.0
During the three and six months ended June 30, 2019, there were 41,704 stock options exercised, net of 16,104 shares tendered in payment of option price and income tax withholding amounts, and 98,686 stock options exercised, net of 35,108 shares tendered in payment of option price and income tax withholding amounts, respectively. There were 60,852 stock options exercised, net of 4,260 shares tendered in payment of option price and income tax withholding amounts, and 127,998 stock options exercised, net of	0.9	0.9
Stock-based compensation expense	1.8	1.8
Common cash dividend declared, $0.31 for the three months ended March 31 and June 30, 2019, respectively. There was $0.28 per share declared for the same periods in 2018	(15.8)		(15.8)
Equity, ending balance at Jun. 30, 2018	1,458.3	690.7	802.6	(35.0)
Equity, beginning balance at Dec. 31, 2018	1,693.9	866.7	851.8	(24.6)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	79.5		79.5
Other comprehensive income (loss), net of tax expense	35.1			35.1
Common stock transactions:
During the three and six months ended June 30, 2019, there were 607 common shares purchased and retired, and 39,360 common shares purchased and retired, respectively. There were 896 common shares purchased and retired, and 22,596 common shares purchased and retired, during the same periods in 2018	(2.5)	(2.5)
During the three and six months ended June 30, 2019, there were 4,356,498 common shares issued, and 4,356,498 common shares issued, respectively. There were 11,389 common shares issued, and 11,389 common shares issued, during the same periods in 2018	176.1	176.1
During the three and six months ended June 30, 2019, there were 11,136 non-vested common shares issued, and 211,164 non-vested common shares issued, respectively. There were 204,991 non-vested common shares issued during the six months ended June 30, 2018	0.0	0.0
During the three and six months ended June 30, 2019, there were 16,032 non-vested common shares forfeited or canceled, and 21,417 non-vested common shares forfeited or canceled, respectively. There were 10,566 non-vested common shares forfeited or canceled, and 27,147 non-vested common shares forfeited or canceled, during the same periods in 2018	0.0	0.0
During the three and six months ended June 30, 2019, there were 41,704 stock options exercised, net of 16,104 shares tendered in payment of option price and income tax withholding amounts, and 98,686 stock options exercised, net of 35,108 shares tendered in payment of option price and income tax withholding amounts, respectively. There were 60,852 stock options exercised, net of 4,260 shares tendered in payment of option price and income tax withholding amounts, and 127,998 stock options exercised, net of	0.7	0.7
Stock-based compensation expense	4.6	4.6
Common cash dividend declared, $0.31 for the three months ended March 31 and June 30, 2019, respectively. There was $0.28 per share declared for the same periods in 2018	(38.8)		(38.8)
Equity, ending balance at Jun. 30, 2019	1,948.6	1,045.6	892.5	10.5
Equity, beginning balance at Mar. 31, 2019	1,736.9	866.6	874.7	(4.4)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	37.9		37.9
Other comprehensive income (loss), net of tax expense	14.9			14.9
Common stock transactions:
During the three and six months ended June 30, 2019, there were 607 common shares purchased and retired, and 39,360 common shares purchased and retired, respectively. There were 896 common shares purchased and retired, and 22,596 common shares purchased and retired, during the same periods in 2018	(0.1)	(0.1)
During the three and six months ended June 30, 2019, there were 4,356,498 common shares issued, and 4,356,498 common shares issued, respectively. There were 11,389 common shares issued, and 11,389 common shares issued, during the same periods in 2018	176.1	176.1
During the three and six months ended June 30, 2019, there were 11,136 non-vested common shares issued, and 211,164 non-vested common shares issued, respectively. There were 204,991 non-vested common shares issued during the six months ended June 30, 2018	0.0	0.0
During the three and six months ended June 30, 2019, there were 16,032 non-vested common shares forfeited or canceled, and 21,417 non-vested common shares forfeited or canceled, respectively. There were 10,566 non-vested common shares forfeited or canceled, and 27,147 non-vested common shares forfeited or canceled, during the same periods in 2018	0.0	0.0
During the three and six months ended June 30, 2019, there were 41,704 stock options exercised, net of 16,104 shares tendered in payment of option price and income tax withholding amounts, and 98,686 stock options exercised, net of 35,108 shares tendered in payment of option price and income tax withholding amounts, respectively. There were 60,852 stock options exercised, net of 4,260 shares tendered in payment of option price and income tax withholding amounts, and 127,998 stock options exercised, net of	0.3	0.3
Stock-based compensation expense	2.7	2.7
Common cash dividend declared, $0.31 for the three months ended March 31 and June 30, 2019, respectively. There was $0.28 per share declared for the same periods in 2018	(20.1)		(20.1)
Equity, ending balance at Jun. 30, 2019	$ 1,948.6	$ 1,045.6	$ 892.5	$ 10.5