Long-Term Debt - Additional Information (Detail) (USD $)	3 Months Ended	9 Months Ended						9 Months Ended		3 Months Ended
	Sep. 30, 2012	Sep. 30, 2012	Sep. 30, 2011	May 01, 2012	Dec. 31, 2011	Dec. 31, 2011 Subsequent Event [Member]	Oct. 31, 2012 Subsequent Event [Member] Private Activity Bonds [Member]	Oct. 31, 2012 Subsequent Event [Member] Private Activity Bonds [Member] Minimum [Member]	Oct. 31, 2012 Subsequent Event [Member] Private Activity Bonds [Member] Maximum [Member]	Sep. 30, 2012 Trust
Debt Instrument [Line Items]
Proceeds Initially kept in trust		$ 68,746,000	$ 0							$ 68,746
Debt assumed in an acquisition	25,215,000	25,215,000		25,215,000
Notices of redemption issued, outstading amount	5,188,696	5,188,696				5,339,943	129,000
Debt maturity	2017							2022	2032
Interest rate								5.00%	5.25%
Fixed-rate debt	100,000	100,000
Fixed-rate debt	6.085%	6.085%
Derivative description of variable rate basis	LIBOR
Derivative, Basis Spread on Variable Rate	3.422%	3.422%
Notional amount variable interest rate swap fair value hedge outstanding	$ 100,000	$ 100,000			$ 100,000