Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Stock-based compensation expense		$ (64.1)	$ (48.6)
Stockholders' Equity Attributable to Parent	$ 2,013.9	1,693.9
Net income	$ 181.0	$ 160.2	$ 106.5
Common stock transactions:
Stock options exercised (in shares)	191,193
Dividends declared (in usd per share)	$ 1.24	$ 1.12	$ 0.96
Common Stock
Stock-based compensation expense			$ 0.0
Stockholders' Equity Attributable to Parent	$ 1,049.3
Net income	$ 0.0		$ 0.0
Common stock transactions:
Common shares purchased and retired	43,560	24,271	33,063
Stock issued during period, new issues (in shares)	4,356,973	3,848,929	11,267,676
Stock issued during period, RSU (in shares)	212,587	214,892	140,246
Stock issued during period, RSU forfeited (in shares)	46,198	43,079	53,571
Stock options exercised (in shares)	143,222	161,217	218,095
Shares tendered in payment of option price and income tax withholding amounts, shares	47,971	38,450	67,792
Retained Earnings [Member]
Stock-based compensation expense		$ (64.1)	$ (48.6)
Stockholders' Equity Attributable to Parent	$ 181.0
Net income	181.0		106.5
Accumulated Other Comprehensive Income (Loss)
Stock-based compensation expense		0.0	0.0
Stockholders' Equity Attributable to Parent	11.0	(24.6)
Net income	$ 0.0		$ 0.0
Restricted Stock [Member]
Stock-based compensation expense		$ 0.0