Income Taxes (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Income before income taxes	$ 116,682	$ 134,601	$ 110,173	$ 118,645	$ 107,662	$ 122,814	$ 121,591	$ 108,066	$ 480,101	$ 460,133	$ 389,997
Tax Act, provisional tax benefit											7,600
Tax Act, measurement period adjustment, tax benefit						$ 1,200
AMT credit carryforward	1,338				1,395				1,338	1,395
Unrecognized tax benefits	8,600								8,600
Interest income accrued on unrecognized tax benefits	1,700				$ 1,100				1,700	1,100
Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward	$ 19,100								19,100
Geographic distribution, foreign
Operating Loss Carryforwards [Line Items]
Income before income taxes									$ 12,200	$ 5,300	$ 7,800