Shareholders' Equity - Summary of Share Count and Par Value Data Related to Shareholders' Equity (Details) - $ / shares	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Preferred Stock
Preferred stock, par value per share	$ 0		$ 0		$ 0
Preferred stock, shares authorized	25,000,000		25,000,000		25,000,000
Preferred stock, shares outstanding	0		0		0
Common Stock
Common stock, par value per share	$ 0		$ 0		$ 0
Common stock, shares authorized	75,000,000		75,000,000		75,000,000
Common stock, shares issued	56,923,369		56,923,369		56,786,849
Common stock, shares outstanding	32,707,281	33,087,043	32,707,281	33,087,043	32,750,727	32,938,466
Treasury stock
Treasury stock, shares held	24,216,088		24,216,088		24,036,122
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	0	0	108,894	72,658