Income Taxes (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Loss Carryforwards [Line Items]
Federal tax rate, percent		21.00%	35.00%
Income tax benefit from the Tax Act			$ 39,300,000
Income tax provision		$ 109,106,000	$ 90,699,000	$ 111,199,000
Actual income tax rates		27.60%	28.50%	37.50%
Income tax expense, foreign minimum tax		$ 600,000
Cash and cash equivalents		363,907,000	$ 467,430,000
Unrecognized income tax benefits		0	841,000	$ 3,982,000
Accrued interest expense related to unrecognized income tax benefits		0	100,000
Interest expense related to unrecognized income tax benefits recognized		0	100,000
Reversal of accrued interest expense related to unrecognized income tax benefits		100,000	500,000
Unrecognized tax benefits, decrease resulting from settlements with taxing authorities	$ 3,300,000
Deferred tax liabilities, net		70,848,000	64,697,000
Valuation allowance for deferred tax assets		3,855,000	3,825,000
Other Assets
Operating Loss Carryforwards [Line Items]
Deferred tax liabilities, net		4,700,000	10,000,000
Other Long-Term Obligations
Operating Loss Carryforwards [Line Items]
Deferred tax liabilities, net		75,500,000	$ 74,700,000
United Kingdom Subsidiary
Operating Loss Carryforwards [Line Items]
Trading losses		4,200,000
Non-US [Member] | Subsidiaries [Member]
Operating Loss Carryforwards [Line Items]
Undistributed earnings of foreign subsidiaries		54,600,000
Cash and cash equivalents		$ 60,600,000