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Debt (Narrative) (Details) - USD ($) |  | 3 Months Ended | 6 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Aug. 03, 2016 | Mar. 31, 2017 | Dec. 31, 2016 | Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2016 | Dec. 31, 2017 | Dec. 30, 2016
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Interest rate description |  |  |  | Borrowings under the 2016 Credit Agreement bear interest at (1) a base rate plus a margin of 0.00% to 0.75%, based on certain financial tests, or (2) United States dollar LIBOR (2.09% and 2.33% at June 30, 2018 for our 2016 Revolving Credit Facility and our 2016 Term Loan, respectively) plus 1.00% to 1.75%, based on certain financial tests. The base rate is determined by the greater of (a) the prime commercial lending rate announced by Bank of Montreal from time to time (5.00% at June 30, 2018), (b) the federal funds effective rate, plus ½ of 1.00%, (c) the daily one month LIBOR rate, plus 1.00%, or (d) 0.00%. The interest rates in effect at June 30, 2018 were 3.09% and 3.33% for our 2016 Revolving Credit Facility and our 2016 Term Loan, respectively. Fees for letters of credit issued under the 2016 Revolving Credit Facility range from 1.00% to 1.75% of the respective face amounts of outstanding letters of credit and are computed based on certain financial tests. |  |  |  |
Repayments of long-term debt (in US dollars) |  |  |  | $ 7,634,000 | $ 7,601,000 |  |  |
Letters of credit outstanding (in US dollars) |  |  |  | 110,300,000 |  |  | $ 110,100,000 |
Borrowings under revolving credit facility (in US dollars) |  |  |  | $ 25,000,000 |  |  | 25,000,000 |
2016 Credit Agreement |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Credit agreement, initiation date | Aug. 03, 2016 |  |  |  |  |  |  |
Expiration date of credit agreement |  |  |  | Aug. 03, 2021 |  |  |  |
2016 Revolving Credit Facility |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Interest rate |  |  |  | 3.09% |  |  |  |
Borrowings under revolving credit facility (in US dollars) |  |  |  | $ 25,000,000 |  |  | 25,000,000 |
2016 Term Loan |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Term Loan (in US dollars) | $ 400,000,000 |  |  | $ 277,215,000 |  |  | $ 284,810,000 |
Interest rate |  |  |  | 3.33% |  |  |  |
Periodic principal payment on term loan (in US dollars) |  | $ 3,800,000 | $ 5,000,000 |  |  | $ 5,000,000 |  |
Term loan, annual principal payments (in US dollars) |  |  |  |  |  |  |  | $ 100,000,000
Repayments of long-term debt (in US dollars) |  |  |  |  |  | $ 95,000,000 |  |
Base Rate [Member] | Minimum [Member] | 2016 Credit Agreement |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Basis spread on variable rate |  |  |  | 0.00% |  |  |  |
Base Rate [Member] | Maximum [Member] | 2016 Credit Agreement |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Basis spread on variable rate |  |  |  | 0.75% |  |  |  |
Prime Rate, Bank of Montreal [Member] | 2016 Credit Agreement |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Commercial lending rate |  |  |  | 5.00% |  |  |  |
Credit Agreement Base Rate, Daily One Month LIBOR Rate [Member] | 2016 Credit Agreement |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Basis spread on variable rate |  |  |  | 1.00% |  |  |  |
London Interbank Offered Rate (LIBOR) [Member] | 2016 Revolving Credit Facility |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Commercial lending rate |  |  |  | 2.09% |  |  |  |
London Interbank Offered Rate (LIBOR) [Member] | 2016 Term Loan |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Commercial lending rate |  |  |  | 2.33% |  |  |  |
London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member] | 2016 Credit Agreement |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Basis spread on variable rate |  |  |  | 1.00% |  |  |  |
London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member] | 2016 Credit Agreement |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Basis spread on variable rate |  |  |  | 1.75% |  |  |  |
Credit Agreement Base Rate, Federal Funds Rate [Member] | 2016 Credit Agreement |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Basis spread on variable rate |  |  |  | 0.50% |  |  |  |
Credit Agreement, 0% Base Rate [Member] | 2016 Credit Agreement |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Basis spread on variable rate |  |  |  | 0.00% |  |  |  |
Revolving Credit Facility [Member] | 2016 Revolving Credit Facility |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Line of credit facility maximum borrowing capacity (in US dollars) | $ 900,000,000 |  |  |  |  |  |  |
Increase in borrowing capacity (in US dollars) |  |  |  | $ 1,300,000,000 |  |  |  |
Letters of credit maximum borrowing capacity (in US dollars) |  |  |  | $ 300,000,000 |  |  |  |
Commitment fee percentage of unused amount |  |  |  | 0.15% |  |  |  |
Revolving Credit Facility [Member] | Minimum [Member] | 2016 Revolving Credit Facility |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Commitment fee percentage of unused amount |  |  |  | 0.15% |  |  |  |
Letter of credit fees |  |  |  | 1.00% |  |  |  |
Revolving Credit Facility [Member] | Maximum [Member] | 2016 Revolving Credit Facility |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Commitment fee percentage of unused amount |  |  |  | 0.30% |  |  |  |
Letter of credit fees |  |  |  | 1.75% |  |  |  |

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