Fair Value Measurements (Narrative) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
	Feb. 05, 2015	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
Accrued earnouts		$ 12,194	$ 19,779	$ 12,194	$ 19,779	$ 15,086	$ 30,063
Performance targets, maximum potential obligation		21,700		21,700
Impairment charge recognized, related to long-lived assets		4,000	4,000	10,800	7,600
Write-offs relating to long-lived assets and intangibles		2,800	2,400	6,700	5,300
Certificate of deposits		500		500		$ 5,300
Held-to-maturity investments, carrying value		1,000		1,000
Held-to-maturity investments, fair value		1,100		1,100
Not Identified by Segment [Member]
Write-offs relating to long-lived assets and intangibles		1,100		2,900
Former Corporate Headquarters Building [Member]
Write-offs relating to long-lived assets and intangibles		$ 100		$ 1,200
Site Leasing Revenue [Member]
Write-offs relating to long-lived assets and intangibles			$ 1,600		$ 2,300
Revolving Credit Facility [Member] | Minimum [Member]
Basis spread on variable interest rate	1.375%			1.375%
Revolving Credit Facility [Member] | Maximum [Member]
Basis spread on variable interest rate	2.00%			2.00%