Fair Value Measurements (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Accrued earnouts	$ 4,496	$ 12,194	$ 4,496	$ 12,194	$ 7,230	$ 15,086
Performance targets, maximum potential obligation	7,300		7,300		10,200
Impairment charge recognized, related to long-lived assets	14,700	4,000	20,900	10,800
Impairment charge recognized, related to long-lived assets, additions	7,500		7,500
Write-offs relating to long-lived assets and intangibles	3,700	2,800	7,900	6,700
Write-offs and disposal costs relating to long-lived assets and intangibles	2,300		2,300
Disposal costs		100		1,200
Certificate of deposits					500
Treasury securities	200		200		200
Held-to-maturity investments, carrying value	900		900		800
Held-to-maturity investments, fair value	900		900		$ 900
Not Identified by Segment [Member]
Write-offs relating to long-lived assets and intangibles	$ 1,100		$ 3,100
Site Leasing Revenue [Member]
Write-offs relating to long-lived assets and intangibles		$ 1,100		$ 2,900
Revolving Credit Facility [Member] | Minimum [Member]
Basis spread on variable interest rate			1.375%
Revolving Credit Facility [Member] | Maximum [Member]
Basis spread on variable interest rate			2.00%