Long-Term Debt - Schedule of Long Term Debt Outstanding (Parenthetical) (Detail) - USD ($) $ in Millions	Dec. 31, 2015	May. 15, 2014	Sep. 30, 2013	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2008
PGS [Member] | 6.10% Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	6.10%
PGS [Member] | 5.40% Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	5.40%
PGS [Member] | 2.60% Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	2.60%
PGS [Member] | 6.15% Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	6.15%
PGS [Member] | 4.10% Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	4.10%
PGS [Member] | 4.35% Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	4.35%
PGS [Member] | 4.20% Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	4.20%
TECO Finance [Member] | 6.75% Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	6.75%
TECO Finance [Member] | 4.00% Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	4.00%
TECO Finance [Member] | 6.57% Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	6.57%
TECO Finance [Member] | 5.15% Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	5.15%
Tampa Electric Company [Member] | 5.65% Refunding bonds [Member]
Debt Instrument [Line Items]
Stated interest rate	5.65%
Tampa Electric Company [Member] | 5.15% Refunding bonds [Member]
Debt Instrument [Line Items]
Stated interest rate	5.15%
Tampa Electric Company [Member] | 1.5% term rate bonds repurchased in 2011 [Member]
Debt Instrument [Line Items]
Stated interest rate	1.50%
Repurchased bonds, par amount					$ 75.0
Tampa Electric Company [Member] | 5.0% Refunding bonds [Member]
Debt Instrument [Line Items]
Stated interest rate	5.00%
Tampa Electric Company [Member] | 6.25% Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	6.25%
Tampa Electric Company [Member] | 6.10% Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	6.10%
Tampa Electric Company [Member] | 5.40% Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	5.40%
Tampa Electric Company [Member] | 2.60% Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	2.60%
Tampa Electric Company [Member] | 6.55% Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	6.55%
Tampa Electric Company [Member] | 6.15% Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	6.15%
Tampa Electric Company [Member] | 4.10% Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	4.10%
Tampa Electric Company [Member] | 4.35% Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	4.35%	4.35%
Tampa Electric Company [Member] | 4.20% Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	4.20%
Tampa Electric Company [Member] | Variable Rate Bonds Repurchased In 2008 [Member]
Debt Instrument [Line Items]
Repurchased bonds, par amount						$ 20.0
Tampa Electric Company [Member] | Refunding Bonds Repurchased In 2013 [Member]
Debt Instrument [Line Items]
Repurchased bonds, par amount			$ 51.6
Tampa Electric Company [Member] | Refunding Bonds Repurchased In 2012 [Member]
Debt Instrument [Line Items]
Repurchased bonds, par amount				$ 86.0
NMGI [Member] | 2.71 % Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	2.71%
NMGI [Member] | 3.64 % Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	3.64%
NMGC [Member] | 4.87 % Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	4.87%
NMGC [Member] | 3.54 % Notes [Member]
Debt Instrument [Line Items]
Stated interest rate	3.54%