Fair Value Measurements (Narrative) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
	Feb. 05, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
Accrued earnouts		$ 8,819	$ 23,136	$ 8,819	$ 23,136	$ 15,086	$ 30,063
Performance targets, maximum potential obligation		14,300		14,300		23,100
Impairment charge recognized, related to long-lived assets		63,400	6,000	74,200	13,600
Write-offs relating to long-lived assets and intangibles		5,300	4,700	12,000	10,000
Certificate of deposits		500		500		5,300
Treasury securities		200		200		200
Held-to-maturity investments, carrying value		1,000		1,000
Held-to-maturity investments, fair value						$ 1,100
Not Identified by Segment [Member]
Write-offs relating to long-lived assets and intangibles		$ 1,400		4,300
Former Corporate Headquarters Building [Member]
Write-offs relating to long-lived assets and intangibles				$ 1,200
Site Leasing Revenue [Member]
Write-offs relating to long-lived assets and intangibles			$ 1,300		$ 3,600
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%