Common Stock and Earnings Per Share - Summary of Common Stock (Details) - USD ($) shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Summary of Common Stock [Line Items]
Balance, Shares	52,100	51,700
Common Stock [Member]
Summary of Common Stock [Line Items]
Balance, Shares	52,085	51,679	51,509	51,117
Balance, Equity	$ 1,460.9	$ 1,436.7	$ 1,428.5	$ 1,401.4
Employee Stock Purchase Plan, Shares	13	8	11
Employee Stock Purchase Plan, Equity	$ 0.7	$ 0.7	$ 0.8
Invest Direct, Shares	309	38	277
Invest Direct, Equity	$ 18.3	$ 3.0	$ 20.7
Options and Stock Awards, Shares	35	85	57
Options and Stock Awards, Equity	$ 2.2	$ 1.3	$ 2.1
Contributions to RSOP, Shares	49	39	47
Contributions to RSOP, Equity	$ 3.0	$ 3.2	$ 3.5
Equity Issuance Program, Shares	0	0	0
Employer Contributions to Pension - Shares	0	0	0
Equity Issuance Program Shares Authorized			13,600
Equity Issuance Program Shares Available for Issuance	2,900