Earnings Per Share (Details) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Earnings Per Share [Abstract]
Weighted-average basic common shares outstanding (in shares)	175.7	175.3	175.6	175.6
Potential number of shares issuable under the Company's LTSIP (in shares)	0.4	0.8	0.5	0.9
Weighted-average diluted common shares outstanding (in shares)	176.1	176.1	176.1	176.5