Common Stock and Earnings Per Share - Summary of Common Stock (Details) - USD ($) shares in Thousands, $ in Millions		12 Months Ended
	Jan. 17, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Summary of Common Stock [Line Items]
Balance, Shares		49,600	49,100
Received for Sale of Land Inventory, Shares		(100)
Received for Sale of Land Inventory, Equity		$ (8.0)
Acquisition of Non-Controlling Interest, Equity		(6.7)		$ (6.0)
Contributions to Pension, Equity		$ 0.0	$ 0.0	$ 19.5
Common Stock [Member]
Summary of Common Stock [Line Items]
Balance, Shares		49,560	49,075	45,929	41,401
Balance, Equity		$ 1,295.3	$ 1,271.4	$ 1,107.6	$ 885.2
Employee Stock Purchase Program, Shares		16	18	18
Employee Stock Purchase Program, Equity		$ 0.9	$ 0.9	$ 0.8
Invest Direct, Shares		344	383	378
Invest Direct, Equity		$ 20.0	$ 19.0	$ 18.9
Options and Stock Awards, Shares		65	43	78
Options and Stock Awards, Equity		$ 3.7	$ 8.6	$ 8.0
Contributions to RSOP, Shares		60
Contributions to RSOP, Equity		$ 3.3
Equity Issuance Program, Shares		130	1,289	1,851
Equity Issuance Program, Equity		$ 8.0	$ 69.9	$ 90.0
Received for Sale of Land Inventory, Shares		(130)
Received for Sale of Land Inventory, Equity		$ (8.0)
Acquisition of Non-Controlling Interest, Shares		0
Acquisition of Non-Controlling Interest, Equity		$ (4.0)
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	0	396
Contributions to Pension, Equity				$ 19.5
Equity Issuance Program Shares Authorized		13,600
Equity Issuance Program Shares Available for Issuance		3,900
Antidilutive Options to Purchase Shares of Common Stock Excluded from the Computation of Earnings Per Share		0	0	0
Common Stock [Member] | Subsequent Event [Member]
Summary of Common Stock [Line Items]
Contributions to Pension, Shares	200
Contributions to Pension, Equity	$ 13.5