Debt - Narratives (Details) - USD ($)		3 Months Ended
	Aug. 10, 2015	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	May 23, 2016	May 22, 2016
Line of Credit Facility
Line of credit maximum borrowing amount		$ 300,000,000
Debt amortization expense		46,000	$ 46,000
Long-term Line of Credit, Noncurrent		$ 74,000,000		$ 76,300,000
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	$ 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%
Long-term Line of Credit, Noncurrent		$ 74,000,000		$ 76,300,000
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%