CONDENSED CONSOLIDATED BALANCE SHEETS (UNAUDITED) (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
ASSETS
Non-utility property, accumulated depreciation and amortization	$ 22.2	$ 19.0	$ 13.9
Capitalization:
Common stock, par value (in dollars per share)	$ 1.00	$ 1.00	$ 1.00
Common stock, authorized (in shares)	70,000,000.0	70,000,000.0	70,000,000.0
Common stock, issued (in shares)	51,700,000	51,600,000	51,100,000
Common stock, outstanding (in shares)	51,700,000	51,600,000	51,100,000
Spire Missouri
Capitalization:
Common stock, par value (in dollars per share)	$ 1.00	$ 1.00	$ 1.00
Common stock, authorized (in shares)	50,000,000.0	50,000,000.0	50,000,000.0
Common stock, issued (in shares)	24,577	24,577	24,577
Common stock, outstanding (in shares)	24,577	24,577	24,577
Spire Alabama
Capitalization:
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01	$ 0.01
Common stock, authorized (in shares)	3,000,000.0	3,000,000.0	3,000,000.0
Common stock, issued (in shares)	2,000,000.0	2,000,000.0	2,000,000.0
Common stock, outstanding (in shares)	2,000,000.0	2,000,000.0	2,000,000.0
Depositary Shares, each representing a 1/1,000th interest in a share of 5.90% Series A Cumulative Redeemable Perpetual Preferred Stock, par value $25.00 per share
Capitalization:
Preferred stock, par value (in dollars per share)	$ 25.00	$ 25.00	$ 25.00
Preferred stock, depositary shares authorized (in shares)	10,000,000.0	10,000,000.0	10,000,000.0
Preferred stock, depositary shares outstanding (in shares)	10,000,000.0	10,000,000.0	10,000,000.0
Preferred stock, depositary shares issued (in shares)	10,000,000.0	10,000,000.0	10,000,000.0