Acquisitions, Goodwill, and Other Intangible Assets (Narrative) (Details) - USD ($)		1 Months Ended	3 Months Ended	12 Months Ended
	Dec. 01, 2016	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Nov. 30, 2016
Business Acquisition [Line Items]
Holding gain upon acquisition of additional ownership of equity-method investments			$ 17,500,000	$ 0	$ 0	$ 37,100,000
Impairment of intangible assets				0	0	0
Goodwill		$ 556,800,000	$ 564,900,000	556,700,000	564,900,000	556,800,000
Goodwill impairment loss				$ 0	$ 0	$ 0
PitchBook [Member]
Business Acquisition [Line Items]
Percentage of voting interests acquired	78.20%
Percentage of voting interest after subsequent acquisition (percent)	100.00%
Percentage of voting interest before subsequent acquisition (percent)							22.00%
Pro Forma, revenue, actual		4,100,000
Pro Forma, income (expense), actual		$ 7,500,000
Acquisition estimated fair value	$ 235,100,000
Cash paid to acquire the entity	188,200,000
Equity method investments, fair value	46,900,000
Book value prior to business acquisition							$ 9,800,000
Holding gain upon acquisition of additional ownership of equity-method investments	37,100,000
Intangible assets	60,700,000
Deferred tax liability	(12,300,000)
Goodwill	$ 192,000,000