Acquisitions Of Businesses (Details) $ in Thousands									10 Months Ended	12 Months Ended
	Nov. 01, 2017 Company	Mar. 01, 2017 Company	Jan. 04, 2017 Company	Apr. 15, 2016 USD ($) Company	Apr. 01, 2016 Company	Oct. 19, 2015 Company	Oct. 13, 2015 Company	Jun. 01, 2015 Company	Nov. 01, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Goodwill										$ 964,893	$ 979,628	$ 843,170
Amortization periods										9 years 6 months
Number of businesses acquired | Company	1	1	1	1	1	1	1	1
2017Acquisitions [Member]
Business Acquisition [Line Items]
Purchase price									$ 109,300
Working capital acquired										$ 9,800
Other net assets										2,300
Goodwill										40,600
Identifiable intangible assets										$ 56,600
Ardent [Member]
Business Acquisition [Line Items]
Business acquisition, percentage of voting interests acquired				100.00%
Purchase price				$ 201,400
Working capital acquired				34,100
Other net assets				3,900
Goodwill				121,900
Identifiable intangible assets				41,500
Acquired goodwill, tax deductible amount				$ 99,700
Amortization periods										13 years 6 months