Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Millions	Total	Cumulative Effect, Period of Adoption, Adjustment	Common stock [Member]	Treasury Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Retained Earnings [Member] Cumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive Income (Loss) [Member] Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period, Shares				(29,200,000)
Dividends Declared [Table Text Block]	1.76
Balance at beginning of year at Dec. 31, 2017			67,100,000
Balance at beginning of period, Common Stock at Dec. 31, 2017			$ 41.6
Balance at beginning of period at Dec. 31, 2017				$ (1,099.0)	$ 698.0	$ 2,261.7		$ (109.6)
Balance at beginning of period (Retained Earnings [Member]) at Dec. 31, 2017		$ 15.2
Balance at beginning of period (Reclassification out of Accumulated Other Comprehensive Income [Domain]) at Dec. 31, 2017		(19.4)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock, Shares			200,000
Issuance of common stock			$ 0.0
Stock repurchases, Shares	1,500,000
Stock repurchases	$ (115.5)
Share-based compensation effects					8.4
Net income	211.3
Dividends declared ($1.92 in 2020, $1.84 in 2019 and $1.76 in 2018)						69.0
Other comprehensive income (loss)	(35.6)	(35.6)
Balance at end of period, Common Stock, Shares at Dec. 31, 2018			67,300,000
Balance at end of period, Common Stock at Dec. 31, 2018			$ 41.6
Balance at end of period at Dec. 31, 2018	$ 1,788.1			$ (1,214.5)	706.4	2,419.2	$ 2,419.2	(164.6)
Balance at end of period (Retained Earnings [Member]) at Dec. 31, 2018		39.4
Balance at end of period (Reclassification out of Accumulated Other Comprehensive Income [Domain]) at Dec. 31, 2018		0.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period, Shares				(30,700,000)
Dividends Declared [Table Text Block]	1.84
Issuance of common stock, Shares			200,000
Issuance of common stock			$ 0.2
Stock repurchases, Shares	2,000,000.0
Stock repurchases	$ (150.0)
Share-based compensation effects					13.7
Net income	211.2
Dividends declared ($1.92 in 2020, $1.84 in 2019 and $1.76 in 2018)						68.5
Other comprehensive income (loss)	$ 1.0	1.0
Balance at end of period, Common Stock, Shares at Dec. 31, 2019	67,536,794		67,500,000
Balance at end of period, Common Stock at Dec. 31, 2019	$ 41.8		$ 41.8
Balance at end of period at Dec. 31, 2019	$ 1,835.1			$ (1,364.5)	720.1	2,601.3	2,601.3	(163.6)
Balance at end of period (Retained Earnings [Member]) at Dec. 31, 2019		0.0
Balance at end of period (Reclassification out of Accumulated Other Comprehensive Income [Domain]) at Dec. 31, 2019		0.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period, Shares				(32,700,000)
Dividends Declared [Table Text Block]	1.92
Issuance of common stock, Shares			300,000
Issuance of common stock			$ 0.1
Stock repurchases, Shares	0
Stock repurchases	$ 0.0
Share-based compensation effects					15.3
Net income	$ 151.3
Dividends declared ($1.92 in 2020, $1.84 in 2019 and $1.76 in 2018)						$ 70.5
Other comprehensive income (loss)		$ 26.1
Balance at end of period, Common Stock, Shares at Dec. 31, 2020	67,751,074		67,800,000
Balance at end of period, Common Stock at Dec. 31, 2020	$ 41.9		$ 41.9
Balance at end of period at Dec. 31, 2020	$ 1,957.4			$ (1,364.5)	$ 735.4		$ 2,682.1	$ (137.5)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period, Shares				(32,700,000)