Earnings Per Share - Anti-dilutive Shares (Details) - shares shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Anti-dilutive effect of the calculation of net dilutive potential shares of common stock issuable
Net dilutive potential common shares issuable (shares)	174	154	193	154
On exercise of options where exercise price is greater than average market value for the period
Anti-dilutive effect of the calculation of net dilutive potential shares of common stock issuable
Net dilutive potential common shares issuable (shares)	95	154	111	154
On exercise of options and vesting of performance-based restricted stock units
Anti-dilutive effect of the calculation of net dilutive potential shares of common stock issuable
Net dilutive potential common shares issuable (shares)	57		57
On settlement of employee stock purchase plan shares
Anti-dilutive effect of the calculation of net dilutive potential shares of common stock issuable
Net dilutive potential common shares issuable (shares)	22		25