CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Parenthetical)	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
2 3/4 Percent Convertible Senior Notes [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Interest rate on convertible senior notes	2.75%	2.75%	2.75%	2.75%	2.75%
1 Percent Convertible Senior Notes [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Interest rate on convertible senior notes	1.00%	1.00%	1.00%	1.00%	1.00%