Credit Facilities and Long-Term Debt - Lines of Credit - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Nov. 26, 2014
Bank of America [Member]
Line of Credit Facility [Line Items]
Interest on amount outstanding	LIBOR plus 175 to 225 basis points
Debt Instruments Weighted Average Interest Rate	2.15%	2.19%	2.14%
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Maximum borrowing credit facility	30.0
Line of credit maturity date	Apr. 01, 2017
Line of Credit Facility, Covenant Terms	the MPSC stated that any amounts borrowed under this increase in excess of $5.0 million would first require the approval of the MPSC
Revolving Credit Facility [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Maximum borrowing credit facility				10.0
Revolving Credit Facility [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Maximum borrowing credit facility				40.0
Revolving Credit Facility [Member] | Bank of America [Member]
Line of Credit Facility [Line Items]
Outstanding borrowings	28.8	24.5
Debt Instruments Weighted Average Interest Rate	2.45%	2.42%	3.33%
Borrowings Outstanding On Revolving Line Of Credit	23.4