Stock Based Compensation (Tables)

6 Months Ended

Jun. 30, 2017

Stock Based Compensation [Abstract]

Stock Based Compensation Expense

The following table provides information regarding the Company's stock-based compensation expense (in thousands):

	Three months ended June 30,				Six months ended June 30,
	2017		2016		2017		2016
Stock-based compensation	$	22,815	$	20,113	$	43,864	$	39,092
Tax effect recognized in net income		7,985		7,040		15,352		13,682

Weighted Average Assumptions Used to Value Option Grants

	Six months ended June 30,
	2017		2016
Risk-free interest rate (%)		2.05		1.39
Expected option life (years)		5.27		5.20
Expected volatility (%)		18.81		21.63
Expected dividend yield (%)		0.68		0.70