Income Taxes (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Loss Carryforwards [Line Items]
Income before taxes	$ 95,785	$ 104,248	$ 101,946	$ 88,018	$ 88,332	$ 85,054	$ 79,971	$ 78,497	$ 389,997	$ 331,854	$ 251,765
Tax benefit (expense) from stock-based compensation arrangements									6,200	230	(1,900)
Tax Cuts and Jobs Act provisional amount recognized									7,600
AMT credit carryforward	1,199				1,872				1,199	1,872
Unrecognized tax benefits	8,500								8,500
Interest income accrued on unrecognized tax benefits	921				$ 521				921	521
Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 14,600								$ 14,600
Minimum | Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Net operating loss expiration date									Jan. 01, 2028
Maximum | Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Net operating loss expiration date									Dec. 31, 2035
Geographic distribution, foreign
Operating Loss Carryforwards [Line Items]
Income before taxes									$ 7,800	$ 7,000	$ 3,900