Common Stock and Earnings Per Share - Summary of Common Stock (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Class of Stock [Line Items]
Common Stock Outstanding, Shares		45,900,000	41,400,000
Contributions to Pension, Value		$ 19.5	$ 0	$ 0
Common Stock [Member]
Class of Stock [Line Items]
Common Stock Outstanding, Shares		45,929,000	41,401,000	39,377,000	37,513,000
Common Stock Outstanding, Value		1,107.6	885.2	784.7	705.6
Employee Stock Purchase Program, Shares		18,000	16,000	20,000
Employee Stock Purchase Program, Value		0.8	0.7	0.8
Invest Direct, Shares		378,000	395,000	474,000
Invest Direct, Value		18.9	18.5	19.2
Options and Stock Awards, Shares		78,000	301,000	95,000
Options and Stock Awards, Value		8.0	17.9	6.0
Equity Issuance Program, Shares		1,851,000	1,312,000	1,275,000
Equity Issuance Program, Value		90.0	63.4	53.1
Forward Sale Agreement and Issuance, Shares		1,807,000
Forward Sales Agreement and Issuance, Value		85.2
Contributions to Pension, Shares	400,000	396,000	0	0
Contributions to Pension, Value	$ 19.5	$ 19.5
Equity Issuance Program Shares Authorized		9,600,000
Equity Issuance Program Shares Available for Issuance		1,300,000
Antidilutive Options to Purchase Shares of Common Stock Excluded from the Computation of Earnings Per Share			0	200,000