Debt - Narratives (Details) - USD ($)		3 Months Ended		6 Months Ended		12 Months Ended
	Aug. 10, 2015	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	May 23, 2016	May 22, 2016
Line of Credit Facility
Debt amortization expense		$ 46,000	$ 38,000	$ 0	$ 0
Line of Credit | Revolving Credit Facility Due 2020
Line of Credit Facility
Debt instrument, term	5 years
Line of credit maximum borrowing amount	$ 200,000,000	$ 300,000,000		$ 300,000,000		$ 300,000,000	$ 300,000,000	$ 200,000,000
Line of credit facility contingent increase to maximum borrowing capacity							$ 100,000,000
Commitment fee percentage per annum				0.25%		0.25%
Line of Credit | Revolving Credit Facility Due 2020 Swingline Sublimit
Line of Credit Facility
Line of credit maximum borrowing amount	15,000,000
Line of Credit | Revolving Credit Facility Due 2020 Letter Of Credit Sublimit
Line of Credit Facility
Line of credit maximum borrowing amount	$ 10,000,000
Minimum | Line of Credit | Revolving Credit Facility Due 2020
Line of Credit Facility
Commitment fee percentage per annum	0.20%
Maximum | Line of Credit | Revolving Credit Facility Due 2020
Line of Credit Facility
Commitment fee percentage per annum	0.40%