Supplementary Financial Data Supplementary Financial Data (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Supplementary Financial Data [Abstract]
Revenue	$ 468,923	$ 363,457	$ 322,856	$ 263,159	$ 367,964	$ 330,767	$ 281,608	$ 234,841	$ 1,418,395	$ 1,215,180	$ 1,036,782
Gross margin	94,816	78,041	70,261	56,976	73,759	68,509	59,587	50,877
Net Income Available to Common Stockholders	$ 12,056	$ 14,352	$ 12,131	$ 8,349	$ 9,767	$ 12,399	$ 12,335	$ 11,413	$ 46,888	$ 45,914	$ 145,577
Earnings Per Share, Basic	$ 0.49	$ 0.58	$ 0.49	$ 0.34	$ 0.40	$ 0.51	$ 0.50	$ 0.47	$ 1.91	$ 1.88	$ 6.11
Earnings Per Share, Diluted	$ 0.43	$ 0.51	$ 0.43	$ 0.31	$ 0.36	$ 0.44	$ 0.44	$ 0.41	$ 1.68	$ 1.65	$ 5.24
Basic Weighted Average Shares Outstanding	24,649	24,605	24,531	24,514	24,489	24,474	24,470	24,417	24,575	24,463	23,822
Diluted Weighted Average Shares Outstanding	30,107	30,067	30,023	29,975	29,944	29,921	29,913	29,870	30,047	29,912	28,763