Debt - 2015 Term Loan Facility (Details) - USD ($)	1 Months Ended			3 Months Ended		9 Months Ended
	Aug. 31, 2017	May 31, 2017	Apr. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Aug. 31, 2015
Debt Instrument [Line Items]
Gain (loss) on debt extinguishment				$ (314,000)	$ 0	$ (9,830,000)	$ 0
Six-Year Term Loan Facility
Debt Instrument [Line Items]
Debt term	6 years
Aggregate principal amount of debt issued								$ 350,000,000
Annual mandatory prepayment of debt				50.00%		50.00%
Repayment of debt			$ 3,000,000
Repaid outstanding borrowings and accrued interest		$ 321,800,000
Gain (loss) on debt extinguishment		$ (9,700,000)