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Common Stock and Earnings Per Share - Summary of Common Stock (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Summary of Common Stock [Line Items] |  |  |  |
Balance, Shares | 51,500,000 | 51,100,000 |  |
Received for Sale of Land Inventory, Equity |  |  | $ (8.0) |
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests |  |  | 6.7 |
Employer Contributions to Defined Benefit Pension Plans - Common Stock Value | $ 0.0 | $ 13.5 | $ 0.0 |
Common Stock [Member] |  |  |  |
Summary of Common Stock [Line Items] |  |  |  |
Balance, Shares | 51,509,000 | 51,117,000 | 49,560,000 | 49,075,000
Balance, Equity | $ 1,428.5 | $ 1,401.4 | $ 1,295.3 | $ 1,271.4
Employee Stock Purchase Plan, Shares | 11,000 | 12,000 | 16,000 |
Employee Stock Purchase Plan, Equity | $ 0.8 | $ 0.8 | $ 0.9 |
Invest Direct, Shares | 277,000 | 257,000 | 344,000 |
Invest Direct, Equity | $ 20.7 | $ 19.0 | $ 20.0 |
Options and Stock Awards, Shares | 57,000 | 22,000 | 65,000 |
Options and Stock Awards, Equity | $ 2.1 | $ 3.6 | $ 3.7 |
Equity Issuance Program, Shares | 0 | 1,000,000 | 130,000 |
Equity Issuance Program, Equity |  | $ 65.7 | $ 8.0 |
Contributions to RSOP, Shares | 47,000 | 50,000 | 60,000 |
Contributions to RSOP, Equity | $ 3.5 | $ 3.5 | $ 3.3 |
Received for Sale of Land Inventory, Shares |  |  | (130,000) |
Received for Sale of Land Inventory, Equity |  |  | $ (8.0) |
Acquisition of Non-Controlling Interest, Shares |  |  | 0 |
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests |  |  | $ (4.0) |
Employer Contributions to Pension - Shares | 0 | 216,000 | 0 |
Employer Contributions to Defined Benefit Pension Plans - Common Stock Value |  | $ 13.5 |  |
Equity Issuance Program Shares Authorized |  |  | 13,600,000 |
Equity Issuance Program Shares Available for Issuance | 2,900,000 |  |  |
Antidilutive Options to Purchase Shares of Common Stock Excluded from the Computation of Earnings Per Share | 0 | 0 | 0 |
Pension Plan [Member] |  |  |  |
Summary of Common Stock [Line Items] |  |  |  |
Employer Contributions to Defined Benefit Pension Plans - Common Stock Value | $ 0.0 | $ 13.5 | $ 0.0 |

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