Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Conversion of restricted stock units to common stock, treasury shares purchased	71,112	69,307	92,608
Partial termination of capped call options, tax impact		$ 116
Convertible debt extinguishment impact, tax impact		$ 2,144
Tax impact of 2036 Senior Notes conversion liability reclassification	$ 18,025
Treasury Stock | Stock appreciation rights
Settlement of stock appreciation rights in common stock, treasury stock purchased	35,955