Net Income Per Common Share (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Net Income Per Common Share [Abstract]
Net income	$ 44.9	$ 47.0	$ 100.1	$ 98.1	$ 136.7
Basic weighted-average common shares outstanding	44,197,164	44,521,757	44,263,445	45,102,716
Stock options	41,208	55,910	83,759	152,368
Stock awards	0	0	2,521	1,323
Diluted weighted-average common shares outstanding	44,238,372	44,577,667	44,349,725	45,256,407
Net income per basic common share	$ 1.02	$ 1.06	$ 2.26	$ 2.18
Net income per diluted common share	$ 1.01	$ 1.05	$ 2.26	$ 2.17
Antidilutive shares not included in net income per diluted common share calculation	2,479,470	2,264,262	2,199,795	1,903,674