Common Stock (Details) - USD ($)	3 Months Ended	6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2021	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Oct. 10, 2021	Apr. 08, 2021	Dec. 31, 2020
Class of Stock [Line Items]
Net proceeds			$ 210,812,000	$ 90,635,000	$ 259,422,000	$ 119,240,000
Common stock, par (in USD per share)	$ 1	$ 1	$ 1		$ 1				$ 1
Subsequent Event
Class of Stock [Line Items]
Number of preferred stock purchase right, per share of common stock dividend (in shares)							1
Common stock, par (in USD per share)							$ 1
Purchase price per unit (in USD per share)							$ 321.70
Preferred stock purchase price, exercisable term, beneficial ownership percentage benchmark							10.00%
Preferred stock purchase price, exercisable term, beneficial ownership percentage benchmark with passive institutional investor							20.00%
Restricted Stock/Unit Plan, and Management Incentive Plan
Class of Stock [Line Items]
Common stock issued (in shares)			47,500
Dividend Reinvestment and Stock Purchase Plan
Class of Stock [Line Items]
Common stock issued (in shares)			130,000
Common stock issued, amount			$ 8,500,000
Equity Shelf Program
Class of Stock [Line Items]
Sale of stock, amount of common stock offered for sale (up to)	$ 341,819,657	$ 341,819,657	$ 341,819,657		$ 341,819,657			$ 500,000,000
Gross proceeds	87,819,931	158,180,343
Less: agent commissions	(878,199)	(1,581,803)
Net proceeds	$ 86,941,732	$ 156,598,540
Number of shares sold (in shares)	1,251,810	2,302,407
Weighted average price per share (in USD per share)	$ 70.15	$ 68.70