Fair Value Measurements - Carrying Amount, Estimated Fair Values and Categorization Under the Fair Value Hierarchy of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Carrying Amount
Assets:
Cash and cash equivalents	$ 96,235	$ 326,655
Liabilities:
Fixed-rate long-term debt	606,529	499,444
Variable-rate long-term debt	631,666	280,163
Fair Value \| Level 1
Assets:
Cash and cash equivalents	96,235	326,655
Liabilities:
Fixed-rate long-term debt	0	0
Variable-rate long-term debt	0	0
Fair Value \| Level 2
Assets:
Cash and cash equivalents	0	0
Liabilities:
Fixed-rate long-term debt	611,308	474,761
Variable-rate long-term debt	635,946	281,750
Interest rate swaps \| Carrying Amount
Assets:
Derivative asset (liability)	749	0
Interest rate swaps \| Fair Value \| Level 1
Assets:
Derivative asset (liability)	0	0
Interest rate swaps \| Fair Value \| Level 2
Assets:
Derivative asset (liability)	749	0
Foreign exchange forward contracts \| Carrying Amount
Assets:
Derivative asset (liability)	427	0
Foreign exchange forward contracts \| Fair Value \| Level 1
Assets:
Derivative asset (liability)	0	0
Foreign exchange forward contracts \| Fair Value \| Level 2
Assets:
Derivative asset (liability)	$ 427	$ 0

Debt Instrument, Fixed Rate Long Term Debt, Fair Value Disclosure

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- Definition

Debt Instrument, Variable Rate Long Term Debt, Fair Value Disclosure

+ References

No definition available.

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- Definition

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

No definition available.

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- Definition

Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

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