Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Revenue	$ 1,109,435	$ 1,421,374	$ 1,784,401
Operating expenses:
Cost of services (exclusive of depreciation and amortization)	366,696	476,355	647,288
Sales and marketing	263,006	352,740	469,238
General and administrative	156,286	174,286	238,554
Depreciation and amortization	146,523	206,270	266,975
Impairment charges	24,911	5,670	0
Total operating expenses	957,422	1,215,321	1,622,055
Operating income	152,013	206,053	162,346
Other income (expense):
Interest expense	(51,537)	(68,181)	(53,851)
Interest expense, related party	(17,002)	(24,770)	(28,846)
Other components of net periodic pension cost	(42,236)	(53,161)	(516)
Loss on early extinguishment of debt	0	(6,375)	(18,375)
Income before benefit (provision) for income taxes	41,238	53,566	60,758
Benefit (provision) for income taxes	107,983	(18,062)	(8,487)
Net income	$ 149,221	$ 35,504	$ 52,271
Net income per common share:
Basic (in USD per share)	$ 4.73	$ 0.87	$ 0.91
Diluted (in USD per share)	$ 4.42	$ 0.82	$ 0.88
Weighted-average shares used in computing basic and diluted net income per common share:
Basic (in shares)	31,522,845	40,845,128	57,331,622
Diluted (in shares)	33,795,594	43,465,998	59,631,195