Summary of Significant Accounting Policies Recent Accounting Pronouncements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 881
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense	4,978	$ 4,153	$ 3,362
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	700
ChangeinForfeiturePolicy [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	400
UnrecognizedExcessTaxBenefit [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	1,000
Tax benefit recognized from adoption of new standard [Member]
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense	$ 3,600