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