Common Stock and Earnings Per Share - Summary of Common Stock (Details) - USD ($) shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Summary of Common Stock [Line Items]
Balance, Shares	49,100	45,900
Contributions to Pension, Equity	$ 0.0	$ 19.5	$ 0.0
Common Stock [Member]
Summary of Common Stock [Line Items]
Balance, Shares	49,075	45,929	41,401	39,377
Balance, Equity	$ 1,271.4	$ 1,107.6	$ 885.2	$ 784.7
Employee Stock Purchase Program, Shares	18	18	16
Employee Stock Purchase Program, Equity	$ 0.9	$ 0.8	$ 0.7
Invest Direct, Shares	383	378	395
Invest Direct, Equity	$ 19.0	$ 18.9	$ 18.5
Options and Stock Awards, Shares	43	78	301
Options and Stock Awards, Equity	$ 8.6	$ 8.0	$ 17.9
Equity Issuance Program, Shares	1,289	1,851	1,312
Equity Issuance Program, Equity	$ 69.9	$ 90.0	$ 63.4
Forward Sale Agreement and Issuance, Shares	1,413	1,807
Forward Sales Agreement and Issuance, Equity	$ 65.4	$ 85.2
Contributions to Pension, Shares	0	396	0
Contributions to Pension, Equity		$ 19.5
Equity Issuance Program Shares Authorized	13,600
Equity Issuance Program Shares Available for Issuance	4,000