Long-Term Debt (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Apr. 30, 1998
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 1,829,702		$ 1,829,702		$ 1,777,307
Debt Issuance Costs	(20,994)		(20,994)		(23,737)
Net	1,808,708		1,808,708		1,753,570
Less current portion of long-term debt	49,658		49,658		70,046
Debt Issuance Costs, Current, Net	0		0		0
Long-term debt, net of current portion, gross	1,780,044		1,780,044		1,707,261
Long-term debt, net of current portion, debt issuance costs	(20,994)		(20,994)		(23,737)
Long-term debt, net of current portion, net	1,759,050		1,759,050		1,683,524
Amortization of debt issuance costs	1,300	$ 1,500	2,744	$ 2,870
Revolving line of credit | Revolving line of credit borrowings due August 2022, weighted-average interest rate of 3.68% as of June 30, 2018
Debt Instrument [Line Items]
Long-term Debt, Gross	120,000		120,000		0
Debt Issuance Costs	(5,944)		(5,944)		(6,672)
Net	$ 114,056		$ 114,056		(6,672)
Weighted average rate on debt	3.68%		3.68%
Term loan facility | Term Loan Due August 2022
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 1,687,500		$ 1,687,500		1,755,000
Debt Issuance Costs	(15,004)		(15,004)		(17,017)
Net	$ 1,672,496		$ 1,672,496		$ 1,737,983
Weighted average rate on debt	3.67%		3.67%		3.28%
Senior notes | 7.55% senior debentures due April 2028
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 14,645		$ 14,645		$ 14,645
Debt Issuance Costs	(46)		(46)		(48)
Net	$ 14,599		$ 14,599		14,597
Stated rate on debt	7.55%		7.55%			7.55%
Other debt | Various debt instruments with maturities through 2023
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 7,557		$ 7,557		7,662
Debt Issuance Costs	0		0		0
Net	$ 7,557		$ 7,557		$ 7,662