Long-Term Debt - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Long-term debt maturities repayment of principal in next twelve months	$ 8,196,000
Long-term debt maturities repayment of principal in year two	12,139,000
Long-term debt maturities repayment of principal in year three	9,141,000
Long-term debt maturities repayment of principal in year four	9,136,000
Long-term debt maturities repayment of principal in year five	12,037,000
Long-term debt maturities repayment of principal in year five thereafter	59,510,000
Total long-term debt	110,103,000	118,481,000
Principal payments	2,900,000
Unsecured senior notes issue	7,000,000
Conversion price of debt instrument	$ 17.01
Carrying amount of convertible debt instrument	187,000	181,000
Maximum debenture redeemable for cash	200,000
Total debentures redeemed for cash	5,000	2,000
Percentage of equity of total capitalization	40.00%
Fixed charge coverage ratio	1.2
Percentage of additional debt covenant required to maintain	20.00%
Maximum limit on payment of dividends	2.5
Cumulative consolidated net income base	85,100,000
Cumulative net income with restrictions	37,600,000
Cumulative net income free of restrictions	47,500,000
Restricted net assets of consolidated subsidiaries	54,200,000
Percentage of restricted net assets	21.00%
Minimum [Member]
Debt Instrument [Line Items]
Long-term debt, interest percentage	5.28%
Maximum [Member]
Debt Instrument [Line Items]
Long-term debt, interest percentage	6.43%
9.57% bond, due May 1, 2018 [Member]
Debt Instrument [Line Items]
Long-term debt, interest percentage	9.57%
Annual sinking fund payment	909,000
Total long-term debt	5,444,000	6,348,000
Debt instrument, maturity date	May 01, 2018
10.03% bond, due May 1, 2018 [Member]
Debt Instrument [Line Items]
Long-term debt, interest percentage	10.03%
Annual sinking fund payment	500,000
Total long-term debt	2,994,000	3,492,000
Debt instrument, maturity date	May 01, 2018
5.68% note, due June 30, 2026 [Member]
Debt Instrument [Line Items]
Long-term debt, interest percentage	5.68%
Total long-term debt	29,000,000	29,000,000
Debt instrument, maturity date	Jun. 30, 2026
7.83% note, due January 1, 2015 [Member]
Debt Instrument [Line Items]
Long-term debt, interest percentage	7.83%
Total long-term debt	4,000,000	6,000,000
Debt instrument, maturity date	Jan. 01, 2015
Maximum limit on payment of dividends	10.0
Cumulative consolidated net income base	185,300,000
Cumulative net income with restrictions	103,000,000
Cumulative net income free of restrictions	$ 82,300,000
First mortgage bond one [Member]
Debt Instrument [Line Items]
Long-term debt, interest percentage	6.85%
First mortgage bond two [Member]
Debt Instrument [Line Items]
Long-term debt, interest percentage	4.90%