Shareholders' Equity (Tables)

12 Months Ended

Sep. 30, 2021

Stockholders Equity Note [Abstract]

Schedule of Variable Stock Purchase Contract Settlement Rates

Each stock purchase contract obligates the holder to purchase, and Spire to issue and deliver, on March 1, 2024, for a price of fifty dollars in cash, a variable number of shares of its common stock as follows (subject to anti-dilution adjustments).

If the applicable market value* per share of Spire common stock is:	Number of shares to be purchased per stock purchase contract is:
Equal to or greater than $78.6906 (“threshold appreciation price”)	0.6354 (“minimum settlement rate”)
Less than $78.6906, but greater than $64.24	$50.00 ÷ applicable market value*
Less than or equal to $64.24 (“reference price”)	0.7783 (“maximum settlement rate”)

*Based on the volume-weighted average price of Spire common stock during the 20 trading days before settlement.

Schedule of Accumulated Other Comprehensive Income

The components of accumulated other comprehensive income (AOCI), net of income taxes, recognized in the balance sheets at September 30 were as follows:

	Net Unrealized Gain (Loss) on Cash Flow Hedges			Defined Benefit Pension and Other Postretirement Benefit Plans			Net Unrealized Gain (Loss) on Available-for- Sale Debt Securities			Total
Spire
Balance at September 30, 2019	$	(29.0	)	$	(2.4	)	$	0.1		$	(31.3	)
Other comprehensive loss		(9.4	)		(0.5	)		—			(9.9	)
Balance at September 30, 2020		(38.4	)		(2.9	)		0.1			(41.2	)
Other comprehensive income (loss)		46.3			(1.3	)		(0.2	)		44.8
Balance at September 30, 2021	$	7.9		$	(4.2	)	$	(0.1	)	$	3.6
Spire Missouri
Balance at September 30, 2019	$	—		$	(2.4	)	$	—		$	(2.4	)
Other comprehensive loss		—			(0.5	)		—			(0.5	)
Balance at September 30, 2020		—			(2.9	)		—			(2.9	)
Other comprehensive loss		—			(1.3	)		—			(1.3	)
Balance at September 30, 2021	$	—		$	(4.2	)	$	—		$	(4.2	)