Performance-Based Common Stock Warrants - Narrative (Details) - Common Stock Purchase Warrants		6 Months Ended
	Mar. 09, 2016 USD ($) vesting_period $ / shares shares	Jun. 30, 2018 USD ($) shares
Class of Warrant or Right [Line Items]
Number of shares called by warrants (in shares) | shares	725,000	475,000
Exercise price of warrants (in dollars per share) | $ / shares	$ 54.55
Number of vesting periods | vesting_period	3
Term of successive vesting periods	2 years	2 years
Vesting period one	2 years
Vested warrants outstanding (in shares) | shares		175,000
Total vesting period		4 years
Number of warrants vested (in shares) | shares		0
Unrecognized estimated fair value of unvested warrants		$ 2,900,000
Supply Threshold
Class of Warrant or Right [Line Items]
Aggregate level of purchases, $260 million threshold	$ 260,000,000
Aggregate level of purchases, $340 million threshold	$ 340,000,000
Supply threshold amount		$ 680,000,000