CONDENSED CONSOLIDATED BALANCE SHEETS (UNAUDITED) (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2018	Sep. 30, 2017	Jun. 30, 2017
Assets
Non-utility property, accumulated depreciation and amortization	$ 9.7	$ 8.6	$ 8.4
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)	50,700,000	48,300,000	48,300,000
Common stock, outstanding (in shares)	50,700,000	48,300,000	48,300,000
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	50,000,000	50,000,000
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	3,000,000	3,000,000.0
Common stock, issued (in shares)	2,000,000	2,000,000	2,000,000
Common stock, outstanding (in shares)	2,000,000	2,000,000	2,000,000