Balance Sheet Information (Details 3) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Customer Deposits
Customer deposits	$ 38,400,000		$ 32,700,000
Accrued Warranty
Balance as of the beginning of period	4,942,000	8,731,000	8,731,000
Warranties issued during the period	478,000	696,000
Settlements made during the period	(1,341,000)	(1,840,000)
Changes in estimate during the period		327,000
Balance as of the end of period	4,079,000	7,914,000	4,942,000
Amount of reclassification from the beginning balance of accrued warranty to accrued installation			$ 1,047,000