Debt (Details) (USD $)	0 Months Ended		3 Months Ended
	Oct. 15, 2014	Feb. 18, 2014	Dec. 31, 2014	Sep. 30, 2014
Debt Instrument [Line Items]
Debt Instrument Carrying Amount			$ 2,460,000,000	$ 2,460,000,000
Debt Instrument, Unamortized Discount			4,869,000	4,014,000
Long-term debt			2,455,131,000	2,455,986,000
Extinguishment of Debt, Amount	500,000,000
Extinguishment of Debt, Interest Terms	4.95%
Line Of Credit Facility [Line Items]
Line of Credit Facility, Capacity Available for Trade Purchases			1,300,000,000
Long Term Debt Other Disclosures [Abstract]
Authorized Commercial Paper			1,250,000,000
Commercial Paper			550,900,000	196,700,000
Debt and Equity Securities Authorized for Issuance			1,750,000,000.00
Debt And Equity Securities Available For Issuance			845,000,000
Regulated Operations Line of Credit Facilities, Covenant Terms			We are required by the financial covenants in each of these facilities to maintain, at the end of each fiscal quarter, a ratio of total debt to total capitalization of no greater than 70 percent.
Ratio of Total Debt to Total Capital			51.00%
Debt Instrument Covenant Description			In addition to these financial covenants, our credit facilities and public indentures contain usual and customary covenants for our business, including covenants substantially limiting liens, substantial asset sales and mergers. Additionally, our public debt indentures relating to our senior notes and debentures, as well as our revolving credit agreements, each contain a default provision that is triggered if outstanding indebtedness arising out of any other credit agreements in amounts ranging from in excess of $15 million to in excess of $100 million becomes due by acceleration or is not paid at maturity.
Debt Instrument Covenant Compliance			We were in compliance with all of our debt covenants as of December 31, 2014.
Stock Issued During Period, Shares, New Issues		9,200,000
Net proceeds from equity offering		390,200,000
Net Proceeds From Issuance Of Debt	494,000,000
Minimum [Member]
Line Of Credit Facility [Line Items]
Debt Instrument, Covenant, Excess Debt Trigger			15,000,000
Maximum [Member]
Line Of Credit Facility [Line Items]
Debt Instrument, Covenant, Excess Debt Trigger			100,000,000
First Facility Regulated Operations [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity			1,250,000,000
Line of Credit Facility, Maximum Borrowing Capacity with Accordion Feature			1,500,000,000
Second Facility Regulated Operations [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity			25,000,000
Third Facility Regulated Operations [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity			10,000,000
Line of Credit Facility, Remaining Borrowing Capacity			4,100,000
Intercompany Facility Regulated Operations [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity			500,000,000
Line Of Credit Facility Description			which bears interest at the lower of (i) the Eurodollar rate under the five-year revolving credit facility or (ii) the lowest rate outstanding under the commercial paper program
Intercompany Facility Nonregulated Operations [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity			500,000,000
Line Of Credit Facility Description			This facility bears interest at a rate equal to the one-month LIBOR rate plus 3.00 percent or (ii) the rate for AEM's borrowings under its committed credit facility plus 0.75 percent.
Nonregulated Facilities [Member]
Line Of Credit Facility [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity			31,100,000
Regulated Facilities [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity			1,300,000,000
Uncommitted Facility Nonregulated Operations [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity			25,000,000
Committed Facility Nonregulated Operations [Member]
Line Of Credit Facility [Line Items]
Line Of Credit Facility Maximum Borrowing Capacity			15,000,000
Unsecured Senior Notes Due 2014 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount			0	500,000,000
Debt Instrument, Interest Rate, Stated Percentage			4.95%	4.95%
Debt Instrument, Maturity Date			Oct. 15, 2014	Oct. 15, 2014
Unsecured Senior Notes Due 2017 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount			250,000,000	250,000,000
Debt Instrument, Interest Rate, Stated Percentage			6.35%	6.35%
Debt Instrument, Maturity Date			Jun. 15, 2017	Jun. 15, 2017
Unsecured Senior Notes Due 2019 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount			450,000,000	450,000,000
Debt Instrument, Interest Rate, Stated Percentage			8.50%	8.50%
Debt Instrument, Maturity Date			Mar. 15, 2019	Mar. 15, 2019
Unsecured Senior Notes Due 2034 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount			200,000,000	200,000,000
Debt Instrument, Interest Rate, Stated Percentage			5.95%	5.95%
Debt Instrument, Maturity Date			Oct. 15, 2034	Oct. 15, 2034
Unsecured Senior Notes Due 2041 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount			400,000,000	400,000,000
Debt Instrument, Interest Rate, Stated Percentage			5.50%	5.50%
Debt Instrument, Maturity Date			Jun. 15, 2041	Jun. 15, 2041
Unsecured Senior Notes Due 2043 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount			500,000,000	500,000,000
Debt Instrument, Interest Rate, Stated Percentage			4.15%	4.15%
Debt Instrument, Maturity Date			Jan. 15, 2043	Jan. 15, 2043
Unsecured Senior Notes Due 2044 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount	500,000,000		500,000,000	0
Debt Instrument, Interest Rate, Stated Percentage	4.125%		4.125%	0.00%
Debt Instrument, Maturity Date			Oct. 15, 2044	Oct. 14, 2044
Debt Instrument, Interest Rate, Effective Percentage	4.086%
Medium Term Notes Due 2025 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount			10,000,000	10,000,000
Debt Instrument, Interest Rate, Stated Percentage			6.67%	6.67%
Debt Instrument, Maturity Date			Dec. 15, 2025	Dec. 15, 2025
Unsecured Debentures Due 2028 [Member]
Debt Instrument [Line Items]
Debt Instrument Carrying Amount			$ 150,000,000	$ 150,000,000
Debt Instrument, Interest Rate, Stated Percentage			6.75%	6.75%
Debt Instrument, Maturity Date			Jul. 15, 2028	Jul. 15, 2028