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Debt - Line of Credit (Details) - USD ($) | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2017 | Dec. 31, 2017 | Dec. 31, 2016
Line of Credit Facility [Line Items] |  |  |
Long-term line of credit |  |  | $ 300,000,000
Revolving Credit Facility [Member] | Wells Fargo, Merrill Lynch, Pierce Fenner Smith, PNC Capital Markets Credit Facility [Member] |  |  |
Line of Credit Facility [Line Items] |  |  |
Long-term line of credit |  | $ 300,000,000 | 300,000,000
Debt Issuance Costs, Line of Credit Arrangements, Net |  | (1,615,000) | (2,044,000)
Long-term Line of Credit, Net |  | 298,385,000 | 297,956,000
5-Year Tranche [Member] | Revolving Credit Facility [Member] | Wells Fargo, Merrill Lynch, Pierce Fenner Smith, PNC Capital Markets Credit Facility [Member] |  |  |
Line of Credit Facility [Line Items] |  |  |
Long-term line of credit |  | $ 50,000,000 | $ 50,000,000
Stated interest rate (percent) |  | 2.6607% | 1.9167%
7-Year Tranche [Member] | Revolving Credit Facility [Member] | Wells Fargo, Merrill Lynch, Pierce Fenner Smith, PNC Capital Markets Credit Facility [Member] |  |  |
Line of Credit Facility [Line Items] |  |  |
Long-term line of credit |  | $ 100,000,000 | $ 100,000,000
Stated interest rate (percent) |  | 2.9607% | 2.2167%
Interest Rate Swap One [Member] | 5-Year Tranche [Member] | Wells Fargo, Merrill Lynch, Pierce Fenner Smith, PNC Capital Markets Credit Facility [Member] |  |  |
Line of Credit Facility [Line Items] |  |  |
Notional amount |  | $ 90,000,000 |
Interest Rate Swap One [Member] | 5-Year Tranche [Member] | Revolving Credit Facility [Member] | Wells Fargo, Merrill Lynch, Pierce Fenner Smith, PNC Capital Markets Credit Facility [Member] |  |  |
Line of Credit Facility [Line Items] |  |  |
Long-term line of credit |  | $ 90,000,000 | $ 90,000,000
Stated interest rate (percent) |  | 2.651% | 2.651%
Interest Rate Swap Two [Member] | 5-Year Tranche [Member] | Wells Fargo, Merrill Lynch, Pierce Fenner Smith, PNC Capital Markets Credit Facility [Member] |  |  |
Line of Credit Facility [Line Items] |  |  |
Notional amount |  | $ 60,000,000 |
Interest Rate Swap Two [Member] | 5-Year Tranche [Member] | Revolving Credit Facility [Member] | Wells Fargo, Merrill Lynch, Pierce Fenner Smith, PNC Capital Markets Credit Facility [Member] |  |  |
Line of Credit Facility [Line Items] |  |  |
Long-term line of credit |  | $ 60,000,000 | $ 60,000,000
Stated interest rate (percent) |  | 2.6525% | 2.6525%
London Interbank Offered Rate (LIBOR) [Member] | 5-Year Tranche [Member] | Wells Fargo, Merrill Lynch, Pierce Fenner Smith, PNC Capital Markets Credit Facility [Member] |  |  |
Line of Credit Facility [Line Items] |  |  |
Basis spread on variable rate, debt instrument | 1.30% |  |
London Interbank Offered Rate (LIBOR) [Member] | 5-Year Tranche [Member] | Revolving Credit Facility [Member] | Wells Fargo, Merrill Lynch, Pierce Fenner Smith, PNC Capital Markets Credit Facility [Member] |  |  |
Line of Credit Facility [Line Items] |  |  |
Basis spread on variable rate, debt instrument |  | 1.30% | 1.30%
London Interbank Offered Rate (LIBOR) [Member] | 7-Year Tranche [Member] | Wells Fargo, Merrill Lynch, Pierce Fenner Smith, PNC Capital Markets Credit Facility [Member] |  |  |
Line of Credit Facility [Line Items] |  |  |
Basis spread on variable rate, debt instrument | 1.60% |  |
London Interbank Offered Rate (LIBOR) [Member] | 7-Year Tranche [Member] | Revolving Credit Facility [Member] | Wells Fargo, Merrill Lynch, Pierce Fenner Smith, PNC Capital Markets Credit Facility [Member] |  |  |
Line of Credit Facility [Line Items] |  |  |
Basis spread on variable rate, debt instrument |  | 1.60% | 1.60%
London Interbank Offered Rate (LIBOR) [Member] | Interest Rate Swap One [Member] | 5-Year Tranche [Member] | Revolving Credit Facility [Member] | Wells Fargo, Merrill Lynch, Pierce Fenner Smith, PNC Capital Markets Credit Facility [Member] |  |  |
Line of Credit Facility [Line Items] |  |  |
Basis spread on variable rate, derivative |  | 1.30% |
London Interbank Offered Rate (LIBOR) [Member] | Interest Rate Swap Two [Member] | 5-Year Tranche [Member] | Revolving Credit Facility [Member] | Wells Fargo, Merrill Lynch, Pierce Fenner Smith, PNC Capital Markets Credit Facility [Member] |  |  |
Line of Credit Facility [Line Items] |  |  |
Basis spread on variable rate, derivative |  | 1.30% |

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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- Definition Long-term Line of Credit, Net + References No definition available. + Details
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- Definition Percentage points added to the reference rate to compute the variable rate on the debt instrument. + References No definition available. + Details
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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Amount, after accumulated amortization, of debt issuance costs related to line of credit arrangements. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section S45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=68176171&loc=SL68176184-208336 + Details
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- Definition The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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