Fair Value Measurements - Summary of Carrying Value and Estimated Fair Value of all Debt (Parenthetical) (Details)	Jun. 30, 2018	Aug. 31, 2017	Jun. 30, 2017	Apr. 30, 2014	Feb. 28, 2013
3.75% Convertible Notes
Fair value measurement
Interest rate (as a percent)	3.75%	3.75%	3.75%
3.50% Convertible Notes
Fair value measurement
Interest rate (as a percent)	3.50%		3.50%		3.50%
3.50% Series A Convertible Notes
Fair value measurement
Interest rate (as a percent)	3.50%		3.50%	3.50%