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