Fair Value Measurements - Schedule of Fair Value Assets and Liabilities Measured on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions | Jun. 30, 2019 | Sep. 30, 2018 | Jun. 30, 2018 |
ASSETS | | | |
Effects of netting and cash margin receivables/payables | $ (7.9) | $ (8.4) | $ (6.3) |
Derivative asset after the effects of netting | 56.9 | 48.2 | 36.7 |
LIABILITIES | | | |
Effects of netting and cash margin receivables/payables | (19.6) | (14.8) | (10.1) |
Derivative liability after the effects of netting | 40.4 | 6.2 | 4.7 |
Spire Missouri | | | |
ASSETS | | | |
Effects of netting and cash margin receivables/payables | | (2.7) | (1.7) |
Derivative asset after the effects of netting | | 20.3 | 23 |
U.S. stock/bond mutual funds | Spire Missouri | | | |
ASSETS | | | |
Effects of netting and cash margin receivables/payables | 0 | 0 | 0 |
Derivative asset after the effects of netting | 20.4 | 20.3 | 23 |
NYMEX/ICE natural gas contracts | Spire Missouri | | | |
ASSETS | | | |
Effects of netting and cash margin receivables/payables | | (2.7) | (1.7) |
Derivative asset after the effects of netting | | 0 | 0 |
LIABILITIES | | | |
Effects of netting and cash margin receivables/payables | (11.1) | (1.9) | (0.2) |
Derivative liability after the effects of netting | 0 | 0 | 0 |
Quoted Prices in Active Markets (Level 1) | | | |
ASSETS | | | |
Derivative asset before effects of netting | 37.8 | 32.1 | 20.8 |
LIABILITIES | | | |
Derivative liability before effects of netting | 11.8 | 2.8 | 1.1 |
Quoted Prices in Active Markets (Level 1) | Spire Missouri | | | |
ASSETS | | | |
Derivative asset before effects of netting | | 23 | 20.6 |
Quoted Prices in Active Markets (Level 1) | U.S. stock/bond mutual funds | Spire Missouri | | | |
ASSETS | | | |
Derivative asset before effects of netting | 20.4 | 20.3 | 18.9 |
Quoted Prices in Active Markets (Level 1) | NYMEX/ICE natural gas contracts | Spire Missouri | | | |
ASSETS | | | |
Derivative asset before effects of netting | | 2.7 | 1.7 |
LIABILITIES | | | |
Derivative liability before effects of netting | 11.1 | 1.9 | 0.2 |
Significant Observable Inputs (Level 2) | | | |
ASSETS | | | |
Derivative asset before effects of netting | 27 | 24.5 | 22.2 |
LIABILITIES | | | |
Derivative liability before effects of netting | 48 | 18 | 13.7 |
Significant Observable Inputs (Level 2) | Spire Missouri | | | |
ASSETS | | | |
Derivative asset before effects of netting | | 0 | 4.1 |
Significant Observable Inputs (Level 2) | U.S. stock/bond mutual funds | Spire Missouri | | | |
ASSETS | | | |
Derivative asset before effects of netting | 0 | 0 | 4.1 |
Significant Observable Inputs (Level 2) | NYMEX/ICE natural gas contracts | Spire Missouri | | | |
ASSETS | | | |
Derivative asset before effects of netting | | 0 | 0 |
LIABILITIES | | | |
Derivative liability before effects of netting | 0 | 0 | 0 |
Significant Unobservable Inputs (Level 3) | | | |
ASSETS | | | |
Derivative asset before effects of netting | 0 | 0 | 0 |
LIABILITIES | | | |
Derivative liability before effects of netting | 0.2 | 0.2 | 0 |
Significant Unobservable Inputs (Level 3) | Spire Missouri | | | |
ASSETS | | | |
Derivative asset before effects of netting | | 0 | 0 |
Significant Unobservable Inputs (Level 3) | U.S. stock/bond mutual funds | Spire Missouri | | | |
ASSETS | | | |
Derivative asset before effects of netting | 0 | 0 | 0 |
Significant Unobservable Inputs (Level 3) | NYMEX/ICE natural gas contracts | Spire Missouri | | | |
ASSETS | | | |
Derivative asset before effects of netting | | 0 | 0 |
LIABILITIES | | | |
Derivative liability before effects of netting | 0 | 0 | 0 |
Corporate, Non-Segment | U.S. stock/bond mutual funds | | | |
ASSETS | | | |
Effects of netting and cash margin receivables/payables | 0 | 0 | |
Derivative asset after the effects of netting | 16.6 | 8.9 | |
Corporate, Non-Segment | Interest rate swaps | | | |
ASSETS | | | |
Effects of netting and cash margin receivables/payables | | 0 | 0 |
Derivative asset after the effects of netting | | 3 | 1.6 |
LIABILITIES | | | |
Effects of netting and cash margin receivables/payables | 0 | | |
Derivative liability after the effects of netting | 27.4 | | |
Corporate, Non-Segment | Quoted Prices in Active Markets (Level 1) | U.S. stock/bond mutual funds | | | |
ASSETS | | | |
Derivative asset before effects of netting | 16.6 | 8.9 | |
Corporate, Non-Segment | Quoted Prices in Active Markets (Level 1) | Interest rate swaps | | | |
ASSETS | | | |
Derivative asset before effects of netting | | 0 | 0 |
LIABILITIES | | | |
Derivative liability before effects of netting | 0 | | |
Corporate, Non-Segment | Significant Observable Inputs (Level 2) | U.S. stock/bond mutual funds | | | |
ASSETS | | | |
Derivative asset before effects of netting | 0 | 0 | |
Corporate, Non-Segment | Significant Observable Inputs (Level 2) | Interest rate swaps | | | |
ASSETS | | | |
Derivative asset before effects of netting | | 3 | 1.6 |
LIABILITIES | | | |
Derivative liability before effects of netting | 27.4 | | |
Corporate, Non-Segment | Significant Unobservable Inputs (Level 3) | U.S. stock/bond mutual funds | | | |
ASSETS | | | |
Derivative asset before effects of netting | 0 | 0 | |
Corporate, Non-Segment | Significant Unobservable Inputs (Level 3) | Interest rate swaps | | | |
ASSETS | | | |
Derivative asset before effects of netting | | 0 | 0 |
LIABILITIES | | | |
Derivative liability before effects of netting | 0 | | |
Gas Utility | Operating Segments | U.S. stock/bond mutual funds | | | |
ASSETS | | | |
Effects of netting and cash margin receivables/payables | 0 | 0 | 0 |
Derivative asset after the effects of netting | 20.4 | 20.3 | 23 |
Gas Utility | Operating Segments | NYMEX/ICE natural gas contracts | | | |
ASSETS | | | |
Effects of netting and cash margin receivables/payables | | (2.7) | (1.7) |
Derivative asset after the effects of netting | | 0 | 0 |
LIABILITIES | | | |
Effects of netting and cash margin receivables/payables | (11.1) | (1.9) | (0.2) |
Derivative liability after the effects of netting | 0 | 0 | 0 |
Gas Utility | Operating Segments | Quoted Prices in Active Markets (Level 1) | U.S. stock/bond mutual funds | | | |
ASSETS | | | |
Derivative asset before effects of netting | 20.4 | 20.3 | 18.9 |
Gas Utility | Operating Segments | Quoted Prices in Active Markets (Level 1) | NYMEX/ICE natural gas contracts | | | |
ASSETS | | | |
Derivative asset before effects of netting | | 2.7 | 1.7 |
LIABILITIES | | | |
Derivative liability before effects of netting | 11.1 | 1.9 | 0.2 |
Gas Utility | Operating Segments | Significant Observable Inputs (Level 2) | U.S. stock/bond mutual funds | | | |
ASSETS | | | |
Derivative asset before effects of netting | 0 | 0 | 4.1 |
Gas Utility | Operating Segments | Significant Observable Inputs (Level 2) | NYMEX/ICE natural gas contracts | | | |
ASSETS | | | |
Derivative asset before effects of netting | | 0 | 0 |
LIABILITIES | | | |
Derivative liability before effects of netting | 0 | 0 | 0 |
Gas Utility | Operating Segments | Significant Unobservable Inputs (Level 3) | U.S. stock/bond mutual funds | | | |
ASSETS | | | |
Derivative asset before effects of netting | 0 | 0 | 0 |
Gas Utility | Operating Segments | Significant Unobservable Inputs (Level 3) | NYMEX/ICE natural gas contracts | | | |
ASSETS | | | |
Derivative asset before effects of netting | | 0 | 0 |
LIABILITIES | | | |
Derivative liability before effects of netting | 0 | 0 | 0 |
Gas Marketing | Operating Segments | NYMEX/ICE natural gas contracts | | | |
ASSETS | | | |
Effects of netting and cash margin receivables/payables | (5.5) | (4.2) | (2.6) |
Derivative asset after the effects of netting | 0.3 | 0 | 0 |
LIABILITIES | | | |
Effects of netting and cash margin receivables/payables | (6.1) | (11.4) | (7.9) |
Derivative liability after the effects of netting | 0 | 0 | 0 |
Gas Marketing | Operating Segments | Natural gas commodity contracts | | | |
ASSETS | | | |
Effects of netting and cash margin receivables/payables | (2.4) | (1.5) | (2) |
Derivative asset after the effects of netting | 19.6 | 16 | 12.1 |
LIABILITIES | | | |
Effects of netting and cash margin receivables/payables | (2.4) | (1.5) | (2) |
Derivative liability after the effects of netting | 13 | 6.2 | 4.7 |
Gas Marketing | Operating Segments | Quoted Prices in Active Markets (Level 1) | NYMEX/ICE natural gas contracts | | | |
ASSETS | | | |
Derivative asset before effects of netting | 0.8 | 0.2 | 0.2 |
LIABILITIES | | | |
Derivative liability before effects of netting | 0.7 | 0.9 | 0.9 |
Gas Marketing | Operating Segments | Quoted Prices in Active Markets (Level 1) | Natural gas commodity contracts | | | |
ASSETS | | | |
Derivative asset before effects of netting | 0 | 0 | 0 |
LIABILITIES | | | |
Derivative liability before effects of netting | 0 | 0 | 0 |
Gas Marketing | Operating Segments | Significant Observable Inputs (Level 2) | NYMEX/ICE natural gas contracts | | | |
ASSETS | | | |
Derivative asset before effects of netting | 5 | 4 | 2.4 |
LIABILITIES | | | |
Derivative liability before effects of netting | 5.4 | 10.5 | 7 |
Gas Marketing | Operating Segments | Significant Observable Inputs (Level 2) | Natural gas commodity contracts | | | |
ASSETS | | | |
Derivative asset before effects of netting | 22 | 17.5 | 14.1 |
LIABILITIES | | | |
Derivative liability before effects of netting | 15.2 | 7.5 | 6.7 |
Gas Marketing | Operating Segments | Significant Unobservable Inputs (Level 3) | NYMEX/ICE natural gas contracts | | | |
ASSETS | | | |
Derivative asset before effects of netting | 0 | 0 | 0 |
LIABILITIES | | | |
Derivative liability before effects of netting | 0 | 0 | 0 |
Gas Marketing | Operating Segments | Significant Unobservable Inputs (Level 3) | Natural gas commodity contracts | | | |
ASSETS | | | |
Derivative asset before effects of netting | 0 | 0 | 0 |
LIABILITIES | | | |
Derivative liability before effects of netting | $ 0.2 | $ 0.2 | $ 0 |