Common Stock and Earnings Per Share - Summary of Common Stock (Details) - USD ($) shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Summary of Common Stock [Line Items]
Balance, Shares	51,700	51,500
Employer Contributions to Defined Benefit Pension Plans - Common Stock Value	$ 0.0	$ 0.0	$ 13.5
Common Stock [Member]
Summary of Common Stock [Line Items]
Balance, Shares	51,679	51,509	51,117	49,560
Balance, Equity	$ 1,436.7	$ 1,428.5	$ 1,401.4	$ 1,295.3
Employee Stock Purchase Plan, Shares	8	11	12
Employee Stock Purchase Plan, Equity	$ 0.7	$ 0.8	$ 0.8
Invest Direct, Shares	38	277	257
Invest Direct, Equity	$ 3.0	$ 20.7	$ 19.0
Options and Stock Awards, Shares	85	57	22
Options and Stock Awards, Equity	$ 1.3	$ 2.1	$ 3.6
Contributions to RSOP, Shares	39	47	50
Contributions to RSOP, Equity	$ 3.2	$ 3.5	$ 3.5
Equity Issuance Program, Shares	0	0	1,000
Equity Issuance Program, Equity			$ 65.7
Employer Contributions to Pension - Shares	0	0	200
Employer Contributions to Defined Benefit Pension Plans - Common Stock Value			$ 13.5
Equity Issuance Program Shares Authorized			13,600
Equity Issuance Program Shares Available for Issuance	2,900
Pension Plan [Member]
Summary of Common Stock [Line Items]
Employer Contributions to Defined Benefit Pension Plans - Common Stock Value	$ 0.0		$ 0.0
Pension Plan [Member] | Common Stock [Member]
Summary of Common Stock [Line Items]
Employer Contributions to Pension - Shares			216
Employer Contributions to Defined Benefit Pension Plans - Common Stock Value			$ 13.5