Income Taxes (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Loss Carryforwards [Line Items]
Income before taxes	$ 107,662	$ 122,814	$ 121,591	$ 108,066	$ 95,785	$ 104,248	$ 101,946	$ 88,018	$ 460,133	$ 389,997	$ 331,854
Tax benefit related to stock-based compensation, recorded to shareholders' equity											230
Tax Act, provisional tax benefit										7,600
Tax Act, measurement period adjustment, tax benefit		$ 1,200							1,200
AMT credit carryforward	1,395				1,199				1,395	1,199
Unrecognized tax benefits	9,300								9,300
Interest income accrued on unrecognized tax benefits	1,100				$ 921				1,100	921
Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 25,500								25,500
Geographic distribution, foreign
Operating Loss Carryforwards [Line Items]
Income before taxes									$ 5,300	$ 7,800	$ 7,000