Significant Accounting Policies - Redeemable Noncontrolling Interest (Details) - USD ($) $ in Thousands	12 Months Ended
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	$ 69,471
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests			$ 17,831
Increase in noncontrolling interests from business acquisitions	8,936	$ (51,488)	50,118
Redeemable Noncontrolling Interest, Equity, Redemption Value	60,180	69,471
Redeemable Noncontrolling Interest [Member]
Redeemable Noncontrolling Interest [Roll Forward]
Redeemable Noncontrolling Interest, Equity, Redemption Value	69,471	194,951	148,534
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests	1,708	155,042	4,820
Increase in noncontrolling interests from business acquisitions	2,274	7,703	13,327
Noncontrolling Interest, Change in Redemption Value	(9,604)	22,809	38,850
Currency translation adjustments	(253)	(950)	(940)
Redeemable Noncontrolling Interest, Equity, Redemption Value	$ 60,180	$ 69,471	$ 194,951

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
-Subparagraph 15
-URI http://asc.fasb.org/extlink&oid=65877616&loc=SL6540498-122764

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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
-Paragraph 1
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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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