Long-Term Debt - Additional Information (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Document Period End Date		Mar. 31, 2019
Long-term debt including current maturities		$ 357,200	$ 327,200
Debt Instrument, Unused Borrowing Capacity, Amount		250,000
Term Note Due January Two Thousand Twenty [Member]
Debt Instrument [Line Items]
Long-term debt including current maturities	[1]	$ 30,000	$ 30,000
Debt Instrument, Interest Rate, Stated Percentage		3.24%	3.23%
Term Note Due February Two Thousand Twenty [Member]
Debt Instrument [Line Items]
Long-term debt including current maturities	[1]	$ 30,000
Debt Instrument, Interest Rate, Stated Percentage		3.24%
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage
3.48% note, due May 31, 2038 [Member]
Debt Instrument [Line Items]
Long-term debt including current maturities		$ 50,000	$ 50,000
3.25% note, due April 30, 2032 [Member]
Debt Instrument [Line Items]
Long-term debt including current maturities		$ 70,000	$ 70,000
Shelf Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		3.98%
MetLife [Member] | Notes Payable, Other Payables [Member] | Shelf Notes [Member]
Debt Instrument [Line Items]
Maturity date term		20 years
New York Life [Member] | Notes Payable, Other Payables [Member] | Shelf Notes [Member]
Debt Instrument [Line Items]
Maturity date term		20 years
Prudential [Member]
Debt Instrument [Line Items]
Senior notes		$ 100,000
Prudential [Member] | Notes Payable, Other Payables [Member] | Shelf Notes [Member]
Debt Instrument [Line Items]
Senior notes		$ 150,000
Maturity date term		20 years
Debt Instrument, Interest Rate, Stated Percentage		3.25%
Prudential [Member] | Shelf Notes [Member]
Debt Instrument [Line Items]
Senior notes		$ 150,000
New York Life [Member]
Debt Instrument [Line Items]
Senior notes		100,000
Debt Instrument, Unused Borrowing Capacity, Amount		50,000
Electric Limited Proceedings [Member]
Debt Instrument [Line Items]
Combined Dollar Amount of Term Notes to Finance Hurricane Michael Costs		$ 60,000

[1] In December 2018, we issued a $30.0 million unsecured term note through PNC Bank N.A. with maturity date of January 21, 2020. The interest rate at March 31, 2019 and December 31, 2018 was 3.24% and 3.23%, respectively, which equals one month LIBOR rate plus 75 basis points.