Warranty Obligations - Narrative (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2012
Product Liability [Line Items]
Warranty obligation	$ 32,307,000	$ 29,366,000	$ 32,307,000	$ 29,366,000	$ 32,969,000	$ 30,432,000	$ 26,777,000	$ 21,338,000
Change in warrant accrual	1,146,000	3,823,000	5,506,000	8,537,000
First and Second Generation Products
Product Liability [Line Items]
Product warranty term				15 years
Third and Fourth Generation Products
Product Liability [Line Items]
Product warranty term				25 years
Failure Rate | Second Generation Products
Product Liability [Line Items]
Increase (Decrease) in Warranty Expense Due to Changes in Warranty Variables	1,100,000	7,300,000	4,200,000	15,900,000
Failure Rate | Third Generation Products
Product Liability [Line Items]
Increase (Decrease) in Warranty Expense Due to Changes in Warranty Variables		(3,100,000)		(3,100,000)
Replacement Cost
Product Liability [Line Items]
Increase (Decrease) in Warranty Expense Due to Changes in Warranty Variables		(400,000)		(100,000)
Replacement Cost | Second Generation Products
Product Liability [Line Items]
Increase (Decrease) in Warranty Expense Due to Changes in Warranty Variables			1,300,000
Claim Rate
Product Liability [Line Items]
Increase (Decrease) in Warranty Expense Due to Changes in Warranty Variables				(4,200,000)
Warranty obligations, fair value option
Product Liability [Line Items]
Warranty obligation fair value measurement portion	$ 3,000,000		$ 3,000,000