Goodwill and Intangible Assets (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Dec. 31, 2017	Sep. 30, 2018
Goodwill [Roll Forward]
Goodwill	$ 2,708,368	$ 2,694,258
Accumulated impairment losses	(429,856)	(433,256)
Balance as of beginning of period		2,278,512
Goodwill acquired, net		(7,222)
Impairment		(3,400)
Other		(6,888)
Balance as of end of period	2,278,512	2,261,002
CPE
Goodwill [Roll Forward]
Goodwill	1,386,680	1,379,339
Accumulated impairment losses	0	0
Balance as of beginning of period		1,386,680
Goodwill acquired, net		0
Impairment		0
Other		(7,341)
Balance as of end of period	1,386,680	1,379,339
N & C
Goodwill [Roll Forward]
Goodwill	1,003,654	1,004,107
Accumulated impairment losses	(429,856)	(433,256)
Balance as of beginning of period		573,798
Goodwill acquired, net		0
Impairment		(3,400)
Other		453
Balance as of end of period	573,798	570,851
Enterprise
Goodwill [Roll Forward]
Goodwill	318,034	310,812
Accumulated impairment losses	0	0
Balance as of beginning of period		318,034
Goodwill acquired, net		(7,222)
Impairment		0
Other		0
Balance as of end of period	$ 318,034	$ 310,812