Basis of Condensed Consolidated Financial Statements (Narrative) (Details) $ / shares in Units, $ in Thousands	1 Months Ended			3 Months Ended			9 Months Ended		12 Months Ended
	Oct. 31, 2020 $ / shares	Jul. 31, 2020 $ / shares	Jan. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) business_day	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) $ / shares	Sep. 14, 2020 $ / shares	Aug. 09, 2020 director	Jun. 26, 2020 $ / shares	Jan. 01, 2020 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Dividends received from investments in affiliates							$ 109	$ 0
Dividends payable (in usd per share) | $ / shares									$ 0.22
Dividends payable									$ 17,400
Dividend equivalents				$ 26,223			43,686		400
Dividends declared (in usd per share) | $ / shares		$ 0.33
Increase in common stock dividend declared		50.00%
Tax escrow deposits				1,400,000			1,400,000		1,400,000
Reserves incorrect disposition of assets				24,800			24,800		22,700
Restricted cash included in other assets				10,129		$ 11,616	10,129	11,616
Restricted cash included in prepaid expenses and other current assets				375		$ 2,002	375	$ 2,002
Tax ASU - Effect of change on retained earnings													$ 16,800
Subsequent Event
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Dividends declared (in usd per share) | $ / shares	$ 0.33
Senator Investment Group, LP and Cannae Holdings, Inc.
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Share price (in usd per share) | $ / shares										$ 66.00		$ 65.00
Increase to original offer (in usd per share) | $ / shares										$ 1.00
Number of directors to be replaced | director											9
Costs accrued							36,900
Compliance Ease
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Proceeds from sale of investments				45,800
Gain on sale of investments				35,100
Accounts Payable and Accrued Liabilities
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Reserves incorrect disposition of assets				$ 10,700			$ 10,700		$ 9,800
Minimum
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Period that escrow deposits are held (in business days) | business_day							2
Maximum
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Period that escrow deposits are held (in business days) | business_day							5
Location, Inc
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Remaining equity interest acquired			66.00%
Consideration transferred			$ 11,500
Investment balance			$ 5,600
Dividends received from investments in affiliates					$ 700
Location, Inc | PIRM
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Ownership percentage			34.00%
Step-up gain			$ 600
Ten Largest Clients | Sales Revenue, Net | Customer Concentration Risk
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Concentration risk, percentage				37.00%		29.00%	35.00%	26.00%