CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDERS' EQUITY - USD ($) $ in Millions	Total	Common Stock Outstanding	Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income/(Loss)	Laclede Gas	Laclede Gas Common Stock Outstanding	Laclede Gas Paid-in Capital	Laclede Gas Retained Earnings	Laclede Gas Accumulated Other Comprehensive Income/(Loss)	Alagasco	Alagasco Common Stock Outstanding	Alagasco Paid-in Capital	Alagasco Retained Earnings
BALANCE (in shares) at Sep. 30, 2012		22,539,431					12,804
BALANCE at Sep. 30, 2012	$ 601.6	$ 22.6	$ 168.6	$ 414.5	$ (4.1)	$ 491.3	$ 0.1	$ 257.3	$ 236.0	$ (2.1)
Stockholders' Equity Rollforward
Net income	52.8			52.8		48.8			48.8
Common stock offering (in shares)		10,005,000					11,745
Common stock offering	427.2	$ 10.0	417.2			477.2		477.2
Dividend reinvestment plan (in shares)		44,074
Dividend reinvestment plan	1.8	$ 0.0	1.8
Stock-based compensation costs	4.4		4.4			3.1		3.1
Equity Incentive Plan (in shares)		108,331
Equity Incentive Plan	2.7	$ 0.1	2.6
Employees’ taxes paid associated with restricted shares withheld upon vesting	(0.9)		(0.9)
Tax benefit - stock compensation	0.6		0.6			0.5		0.5
Dividends declared:
Common stock	(47.2)			(47.2)		(47.0)			(47.0)
Other comprehensive loss, net of tax	3.3				3.3	0.0
BALANCE (in shares) at Sep. 30, 2013		32,696,836					24,549
BALANCE at Sep. 30, 2013	1,046.3	$ 32.7	594.3	420.1	(0.8)	973.9	$ 0.1	738.1	237.8	(2.1)
BALANCE (in shares) at Dec. 31, 2012												1,972,052
BALANCE at Dec. 31, 2012											$ 360.5	$ 0.0	$ 34.5	$ 326.0
Stockholders' Equity Rollforward
Net income											57.4			57.4
Dividends declared:
Common stock											(33.3)			(33.3)
BALANCE (in shares) at Dec. 31, 2013												1,972,052
BALANCE at Dec. 31, 2013											384.6	$ 0.0	34.5	350.1
BALANCE (in shares) at Sep. 30, 2013		32,696,836					24,549
BALANCE at Sep. 30, 2013	1,046.3	$ 32.7	594.3	420.1	(0.8)	973.9	$ 0.1	738.1	237.8	(2.1)
Stockholders' Equity Rollforward
Net income	35.6					35.3					19.8
BALANCE (in shares) at Dec. 31, 2013												1,972,052
BALANCE at Dec. 31, 2013											384.6	$ 0.0	34.5	350.1
BALANCE (in shares) at Sep. 30, 2013		32,696,836					24,549
BALANCE at Sep. 30, 2013	1,046.3	$ 32.7	594.3	420.1	(0.8)	973.9	$ 0.1	738.1	237.8	(2.1)
Stockholders' Equity Rollforward
Net income	84.6			84.6		90.1			90.1
Common stock offering (in shares)		10,350,000					28
Common stock offering	456.8	$ 10.4	446.4			1.1		1.1
Equity units offering	(19.7)		(19.7)
Dividend reinvestment plan (in shares)		33,667
Dividend reinvestment plan	1.5		1.5
Stock-based compensation costs	5.8		5.8			4.2		4.2
Equity Incentive Plan (in shares)		97,902
Equity Incentive Plan	1.7	$ 0.1	1.6
Employees’ taxes paid associated with restricted shares withheld upon vesting	(1.1)		(1.1)
Tax benefit - stock compensation	0.6		0.6			0.6		0.6
Dividends declared:
Common stock	(67.2)			(67.2)		(62.3)			(62.3)
Other comprehensive loss, net of tax	(0.9)				(0.9)	0.2				0.2
BALANCE (in shares) at Sep. 30, 2014		43,178,405					24,577					1,972,052
BALANCE at Sep. 30, 2014	1,508.4	$ 43.2	1,029.4	437.5	(1.7)	1,007.8	$ 0.1	744.0	265.6	(1.9)	849.6	$ 0.0	503.9	345.7
BALANCE (in shares) at Dec. 31, 2013												1,972,052
BALANCE at Dec. 31, 2013											384.6	$ 0.0	34.5	350.1
Stockholders' Equity Rollforward
Net income											33.0			33.0
Dividends declared:
Common stock											(37.4)			(37.4)
Purchase accounting adjustments											469.4		469.4
BALANCE (in shares) at Sep. 30, 2014		43,178,405					24,577					1,972,052
BALANCE at Sep. 30, 2014	1,508.4	$ 43.2	1,029.4	437.5	(1.7)	1,007.8	$ 0.1	744.0	265.6	(1.9)	849.6	$ 0.0	503.9	345.7
Stockholders' Equity Rollforward
Net income	136.9			136.9		105.3			105.3		48.0			48.0
Common stock offering (in shares)												0
Common stock offering											(27.0)		(27.0)
Dividend reinvestment plan (in shares)		31,166
Dividend reinvestment plan	1.6		1.6
Stock-based compensation costs	3.0		3.0			3.7		3.7
Equity Incentive Plan (in shares)		125,441
Equity Incentive Plan	5.1	$ 0.1	5.0
Employees’ taxes paid associated with restricted shares withheld upon vesting	(1.6)		(1.6)
Tax benefit - stock compensation	0.7		0.7			0.5		0.5
Dividends declared:
Common stock	(80.2)			(80.2)		(79.7)			(79.7)
Purchase accounting adjustments											4.0		4.0
Other comprehensive loss, net of tax	(0.3)				(0.3)	0.2				0.2
BALANCE (in shares) at Sep. 30, 2015		43,335,012					24,577					1,972,052
BALANCE at Sep. 30, 2015	1,573.6	$ 43.3	1,038.1	494.2	(2.0)	1,037.8	$ 0.1	748.2	291.2	(1.7)	874.6	$ 0.0	480.9	393.7
Stockholders' Equity Rollforward
Net income	(18.7)					(3.6)					(9.6)
BALANCE (in shares) at Sep. 30, 2015		43,335,012					24,577					1,972,052
BALANCE at Sep. 30, 2015	$ 1,573.6	$ 43.3	$ 1,038.1	$ 494.2	$ (2.0)	$ 1,037.8	$ 0.1	$ 748.2	$ 291.2	$ (1.7)	$ 874.6	$ 0.0	$ 480.9	$ 393.7