Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended			12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Significant Accounting Policies [Line Items]
Provision for income taxes					$ (18,172)	$ (54,524)	$ (57,394)
Deferred income tax (benefit)					(40,769)	18,213	35,110
Accounts payable and accrued expenses			$ 168,284		143,849	168,284
Capitalized data and database costs carrying value			601,025		$ 636,948	601,025
Subscription-based revenues contractual term					10 years
Employee stock purchase plan percent of stock price at closing date					85.00%
Escrow deposits			619,400		$ 961,500	619,400
Reserves incorrect disposition of assets			22,200		21,700	22,200
Share-based compensation true-up (less than)					$ 35,867	39,849	35,786
Maximum [Member]
Significant Accounting Policies [Line Items]
Useful life of other intangible assets, minimum (in years)					20 years
Contractual service period used to recognize revenue					3 years
Escrow deposits, period held					5 years
Maximum [Member] | Buildings [Member]
Significant Accounting Policies [Line Items]
Property and equipment, estimated useful lives, minimum (in years)					40 years
Maximum [Member] | Furniture and equipment [Member]
Significant Accounting Policies [Line Items]
Property and equipment, estimated useful lives, minimum (in years)					10 years
Maximum [Member] | Capitalized software [Member]
Significant Accounting Policies [Line Items]
Property and equipment, estimated useful lives, minimum (in years)					15 years
Maximum [Member] | Database Development Costs [Member]
Significant Accounting Policies [Line Items]
Property and equipment, estimated useful lives, minimum (in years)					20 years
Minimum [Member]
Significant Accounting Policies [Line Items]
Useful life of other intangible assets, minimum (in years)					2 years
Contractual service period used to recognize revenue					1 year
Escrow deposits, period held					2 years
Minimum [Member] | Buildings [Member]
Significant Accounting Policies [Line Items]
Property and equipment, estimated useful lives, minimum (in years)					25 years
Minimum [Member] | Furniture and equipment [Member]
Significant Accounting Policies [Line Items]
Property and equipment, estimated useful lives, minimum (in years)					3 years
Minimum [Member] | Capitalized software [Member]
Significant Accounting Policies [Line Items]
Property and equipment, estimated useful lives, minimum (in years)					3 years
Minimum [Member] | Database Development Costs [Member]
Significant Accounting Policies [Line Items]
Property and equipment, estimated useful lives, minimum (in years)					5 years
Flood data [Member]
Significant Accounting Policies [Line Items]
Capitalized data and database costs carrying value			55,416		$ 55,416	55,416
Restatement Adjustment [Member]
Significant Accounting Policies [Line Items]
Provision for income taxes	$ (3,000)	$ (1,300)	(3,100)				9,400	$ (2,000)
Deferred income tax (benefit)					(4,300)	(3,100)
Interest expense							5,200
Interest expense, net							3,100
Accounting Standards Update 2017-07 [Member]
Significant Accounting Policies [Line Items]
Reclassification of net periodic benefit costs from adoption of ASU 2017-07					800	3,400	$ 500
Accounting Standards Update 2016-09 [Member]
Significant Accounting Policies [Line Items]
Deferred income tax (benefit)					(2,500)
Share-based compensation true-up (less than)					$ (100)
Accounting Standards Update 2014-09 [Member] | Forecast [Member]
Significant Accounting Policies [Line Items]
Impact of adoption of new revenue recognition guidance on deferred revenue				5.00%
Impact of adoption of new revenue recognition guidance on stockholders' equity				(5.00%)
Misclassification Related To Liabilities [Member]
Significant Accounting Policies [Line Items]
Accounts payable and accrued expenses			$ 32,000			$ 32,000
Customer Concentration Risk [Member] | Sales Revenue, Net [Member] | Ten Largest Clients [Member]
Significant Accounting Policies [Line Items]
Concentration risk, percentage					38.70%	43.00%	33.50%
Customer Concentration Risk [Member] | Sales Revenue, Net [Member] | Client A [Member]
Significant Accounting Policies [Line Items]
Concentration risk, percentage					11.10%	14.00%
Customer Concentration Risk [Member] | Sales Revenue, Net [Member] | Client B [Member]
Significant Accounting Policies [Line Items]
Concentration risk, percentage						11.50%