Subsequent Events - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	0 Months Ended
	Feb. 08, 2011 NMGC 2011 Notes [Member]	Oct. 17, 2014 Subsequent Event [Member] TECO Coal [Member]	Oct. 17, 2014 Subsequent Event [Member] TECO Coal [Member] Maximum [Member]
Subsequent Event [Line Items]
Ownership interest sold		$ 120
Contingent payments			50
Aggregate principal amount issued	$ 200
Issuance of notes, maturity year	Feb. 08, 2021