Debt - Additional Information (Details) - USD ($)						3 Months Ended		9 Months Ended		18 Months Ended
	Jul. 31, 2017	Jul. 07, 2017	Apr. 03, 2017	Dec. 02, 2016	Jan. 05, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2017	Apr. 11, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Repayment of debt and revolving credit facility								$ 100,000,000	$ 25,000,000
Interest expense						$ 1,600,000	$ 1,200,000	4,600,000	$ 4,000,000
Wells Fargo Bank, National Association
Debt Instrument [Line Items]
Proceeds from lines of credit								37,000,000
Repayment of debt and revolving credit facility								100,000,000		$ 134,500,000
Carry amount of debt						197,500,000		197,500,000				$ 260,500,000
Fair value of debt						199,200,000		199,200,000
Wells Fargo Bank, National Association | Secured Debt | Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity					$ 400,000,000
Annual capital expenditure limitation per Credit Agreement					35,000,000						$ 45,000,000
Maximum allowed non-permitted investments per Credit Agreement					10,000,000						$ 20,000,000
Deferred issuance cost, gross					$ 7,900,000
Wells Fargo Bank, National Association | Secured Debt | Line of Credit | Credit Agreement, Interest Rate Option One | London Interbank Offered Rate (LIBOR) | Minimum
Debt Instrument [Line Items]
Basis spread on variable rate					1.50%
Wells Fargo Bank, National Association | Secured Debt | Line of Credit | Credit Agreement, Interest Rate Option One | London Interbank Offered Rate (LIBOR) | Maximum
Debt Instrument [Line Items]
Basis spread on variable rate					2.25%
Wells Fargo Bank, National Association | Secured Debt | Line of Credit | Credit Agreement, Interest Rate Option Two | London Interbank Offered Rate (LIBOR) | Minimum
Debt Instrument [Line Items]
Basis spread on variable rate					0.50%
Wells Fargo Bank, National Association | Secured Debt | Line of Credit | Credit Agreement, Interest Rate Option Two | London Interbank Offered Rate (LIBOR) | Maximum
Debt Instrument [Line Items]
Basis spread on variable rate					1.25%
Wells Fargo Bank, National Association | Secured Debt | Line of Credit | Credit Agreement, Interest Rate Option Two | Federal Funds Rate
Debt Instrument [Line Items]
Basis spread on variable rate					0.50%
Wells Fargo Bank, National Association | Revolving Credit Facility
Debt Instrument [Line Items]
Proceeds from lines of credit								37,000,000
Repayment of debt and revolving credit facility								92,500,000
Carry amount of debt						12,500,000		12,500,000				68,000,000
Wells Fargo Bank, National Association | Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity					$ 200,000,000
Debt instrument, term (in years)					5 years
Proceeds from lines of credit	$ 12,000,000	$ 15,000,000	$ 10,000,000	$ 40,000,000	$ 55,000,000
Wells Fargo Bank, National Association | Revolving Credit Facility | Line of Credit | Minimum
Debt Instrument [Line Items]
Unused capacity, percentage					0.20%
Wells Fargo Bank, National Association | Revolving Credit Facility | Line of Credit | Maximum
Debt Instrument [Line Items]
Unused capacity, percentage					0.35%
Wells Fargo Bank, National Association | Term Loan Facility
Debt Instrument [Line Items]
Proceeds from lines of credit								0
Repayment of debt and revolving credit facility								7,500,000
Carry amount of debt						$ 185,000,000		$ 185,000,000				$ 192,500,000
Wells Fargo Bank, National Association | Term Loan Facility | Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity					$ 200,000,000
Debt instrument, term (in years)					5 years
Principal periodic payment, percent, years one and two					5.00%
Principal periodic payment, percent, years three and four					10.00%
Principal periodic payment, percent, year five					15.00%
Proceeds from lines of credit					$ 200,000,000
Wells Fargo Bank, National Association | Letter of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity					$ 10,000,000
Wells Fargo Bank, National Association | Letter of Credit | Line of Credit | Minimum
Debt Instrument [Line Items]
Commitment fee percentage					1.50%
Wells Fargo Bank, National Association | Letter of Credit | Line of Credit | Maximum
Debt Instrument [Line Items]
Commitment fee percentage					2.25%
Wells Fargo Bank, National Association | Bridge Loan
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity					$ 10,000,000