Basis of Condensed Consolidated Financial Statements (Details) - USD ($)		1 Months Ended		3 Months Ended			9 Months Ended
	Jan. 01, 2018	Aug. 31, 2017	Jun. 30, 2017	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Accounts payable and accrued expenses				$ 161,004,000			$ 161,004,000		$ 168,284,000
Provision for income taxes				11,851,000		$ 15,922,000	36,759,000	$ 51,984,000
Equity in earnings/(losses) of affiliates, net of tax (less than)				(229,000)		607,000	(1,232,000)	595,000
Revenue from Related Parties				2,800,000		2,500,000	6,900,000	7,700,000
Costs and Expenses, Related Party				3,200,000		2,900,000	8,900,000	8,400,000
Loss)/gain on Investments and other, net				(3,095,000)		$ (20,056,000)	(6,513,000)	(17,873,000)
Escrow deposit				1,300,000,000			1,300,000,000		619,400,000
Reserves incorrect disposition of assets				21,200,000			21,200,000		22,200,000
Other liabilities				162,494,000			162,494,000		132,043,000
Accumulated Other Comprehensive Income (Loss), Net of Tax				(96,517,000)			(96,517,000)		$ (122,418,000)
Deferred revenue							$ 26,037,000	35,814,000
RELS LLC [Member]
Loss)/gain on Investments and other, net					$ 6,100,000
Pension and Other Postretirement Benefit Contributions			$ 13,500,000
Other liabilities			9,200,000		9,200,000
Accumulated Other Comprehensive Income (Loss), Net of Tax			$ 1,800,000		$ 1,800,000
Minimum [Member]
Escrow deposits, period held by the Company (in business days)							2 days
Maximum [Member]
Equity in earnings/(losses) of affiliates, net of tax (less than)								$ 595,000
Escrow deposits, period held by the Company (in business days)							5 days
Mercury, Inc. [Member]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners			45.00%		45.00%
Payments to Acquire Equity Method Investments			$ 70,000,000
Business Acquisition, Percentage of Voting Interests Acquired		55.00%
Call Option Fair Value			4,600,000		$ 4,600,000
Business Combination, Consideration Transferred		$ 83,000,000
Loss)/gain on Investments and other, net		$ 1,900,000
Restatement Adjustment [Member]
Provision for income taxes				$ 3,000,000			$ 4,300,000
Ten Largest Clients [Member] | Sales Revenue, Net [Member] | Customer Concentration Risk [Member]
Concentration risk, percentage				37.00%		41.00%	39.00%	42.00%
Client A [Member] | Sales Revenue, Net [Member] | Customer Concentration Risk [Member]
Concentration risk, percentage				11.00%		15.00%	12.00%	15.00%
Client B [Member] | Sales Revenue, Net [Member] | Customer Concentration Risk [Member]
Concentration risk, percentage						11.00%	10.00%	12.00%
Accounting Standards Update 2014-09 [Member] | Forecast [Member]
Deferred revenue	$ 0.05
Stockholders' Equity, Period Increase (Decrease)	$ 0.05
Misclassification Related To Liabilities [Member]
Accounts payable and accrued expenses			$ 32,000,000		$ 32,000,000