Consolidated Statements of Stockholders' Equity - USD ($) $ in Millions	Total	Common Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Beginning Balance at Dec. 31, 2015	$ 950.5	$ 311.7	$ 638.4	$ 0.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	95.7		95.7
Other comprehensive loss, net of tax	(8.6)			(8.6)
Common stock transactions:
Common shares purchased and retired	(26.9)	(26.9)
Common shares issued	0.0
Non-vested common shares issued	0.0
Non-vested common shares forfeited or canceled	0.0
Stock options exercised, net of shares tendered in payment of option price and income tax withholding amounts	4.7	4.7
Tax benefit of stock-based compensation	2.2	2.2
Stock-based compensation expense	4.4	4.4
Cash dividends declared:
Common (per share)	(39.4)		(39.4)
Ending Balance at Dec. 31, 2016	982.6	296.1	694.7	(8.2)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	106.5		106.5
Other comprehensive loss, net of tax	(3.8)			(3.8)
Common stock transactions:
Common shares purchased and retired	(1.3)	(1.3)
Non-vested common shares issued	0.0
Non-vested common shares forfeited or canceled	0.0
Stock options exercised, net of shares tendered in payment of option price and income tax withholding amounts	2.4	2.4
Tax benefit of stock-based compensation		2.6
Stock-based compensation expense	3.8	3.8
Cash dividends declared:
Common (per share)	(48.6)		(48.6)
Ending Balance at Dec. 31, 2017	1,427.6	687.0	752.6	(12.0)
Common stock transactions:
Common Stock, Value, Issued	386.0	386.0
Net income	160.2		160.2
Reclassifications Of Income Tax Effects				(3.1)
Other comprehensive loss, net of tax	(9.5)			(9.5)
Common shares purchased and retired	(1.0)	(1.0)
Non-vested common shares issued	0.0
Non-vested common shares forfeited or canceled	0.0
Stock options exercised, net of shares tendered in payment of option price and income tax withholding amounts	1.8	1.8
Tax benefit of stock-based compensation		1.1
Stock-based compensation expense	5.6
Cash dividends declared:
Common (per share)	(64.1)		(64.1)
Ending Balance at Dec. 31, 2018	1,693.9	866.7	$ 851.8	$ (24.6)
Common stock transactions:
Common Stock, Value, Issued	$ 173.3	$ 173.3