Significant Accounting Policies - Redeemable Noncontrolling Interest (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Redeemable Noncontrolling Interest [Line Items]
Consolidated Accounts Receivable Percentage	10.00%	10.00%
Redeemable Noncontrolling Interest [Roll Forward]
Redeemable Noncontrolling Interest, Equity, Redemption Value	$ 60,180
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests	(9,650)
Increase in noncontrolling interests from business acquisitions	6,960	$ 8,936	$ 51,488
Redeemable Noncontrolling Interest, Equity, Redemption Value	$ 62,886	60,180
Redeemable Noncontrolling Interest [Member]
Redeemable Noncontrolling Interest [Roll Forward]
Redeemable Noncontrolling Interest, Equity, Redemption Value		60,180	69,471	$ 194,951
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests		910	1,708	155,042
Increase in noncontrolling interests from business acquisitions		1,666	2,274	7,703
Noncontrolling Interest, Change in Redemption Value		1,498	(9,604)	22,809
Currency translation adjustments		$ 452	(253)	(950)
Redeemable Noncontrolling Interest, Equity, Redemption Value			$ 60,180	$ 69,471

The percentage of accounts receivable that the largest client did not exceed during the period.

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- Definition

Adjustments to redeemable noncontrolling interest resulting from foreign currency translation adjustments.

+ References

No definition available.

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- Definition

Redeemable Noncontrolling Interest [Roll Forward]

+ References

No definition available.

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- Definition

Change in noncontrolling interest during the period as a result of a change in the redemption value of redeemable noncontrolling interest.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 480
-SubTopic 10
-Section S99
-Paragraph 3A
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-URI http://asc.fasb.org/extlink&oid=65877616&loc=SL6540498-122764

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-Publisher FASB
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- Definition

Decrease in noncontrolling interest (for example, but not limited to, redeeming or purchasing the interests of noncontrolling shareholders, issuance of shares (interests) by the non-wholly owned subsidiary to the parent entity for other than cash, and a buyback of shares (interest) by the non-wholly owned subsidiary from the noncontrolling interests).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
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-Publisher FASB
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- Definition

Amount of increase in noncontrolling interest from a business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
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- Definition

Redemption value, as if currently redeemable, of redeemable noncontrolling interest classified as temporary equity and the election has been made to accrete changes in redemption value to the earliest redemption date.

+ References

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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