Supplementary Financial Data Supplementary Financial Data (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplementary Financial Data [Abstract]
Standard and Extended Product Warranty Accrual, Period Increase (Decrease)			$ 8,500			$ 14,500	$ 2,800	$ 2,200	$ 8,500	$ 19,409	$ 0
Revenue	$ 621,702	$ 476,423	456,866	$ 406,980	$ 523,246	442,464	401,247	324,370	1,961,971	1,691,327	1,418,395
Gross margin	115,551	101,750	89,268	86,699	104,586	78,829	81,539	64,198
Net Income Available to Common Stockholders	$ 15,882	$ 18,852	$ 15,770	$ 15,664	$ 19,343	$ 9,724	$ 14,697	$ 7,970	$ 66,168	$ 51,734	$ 46,888
Earnings Per Share, Basic	$ 0.57	$ 0.74	$ 0.63	$ 0.63	$ 0.78	$ 0.39	$ 0.60	$ 0.32	$ 2.57	$ 2.10	$ 1.91
Earnings Per Share, Diluted	$ 0.53	$ 0.64	$ 0.55	$ 0.55	$ 0.67	$ 0.35	$ 0.52	$ 0.30	$ 2.26	$ 1.84	$ 1.68
Basic Weighted Average Shares Outstanding	27,736	25,581	24,990	24,738	24,671	24,669	24,669	24,657	25,769	24,666	24,575
Diluted Weighted Average Shares Outstanding	31,172	30,675	30,619	30,329	30,166	30,139	30,077	30,032	30,688	30,116	30,047
Total valuation adjustments and write-offs	$ 7,681				$ 3,992						$ 3,638