Earnings Per Share (Details)
In Millions, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Mar. 31, 2015
Mar. 31, 2014
Earnings Per Share [Abstract]        
Weighted-average basic common shares outstanding (in shares) 176.2 175.7 176.0 175.6
Potential number of shares issuable under the Company's LTSIP (in shares) 0.3 0.4 0.3 0.5
Weighted-average diluted common shares outstanding (in shares) 176.5 176.1 176.3 176.1