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Shareholders' Equity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands				3 Months Ended		12 Months Ended
	Apr. 17, 2019	Mar. 08, 2019	Feb. 13, 2019	Mar. 31, 2019	Apr. 01, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Feb. 10, 2016
Class of Stock [Line Items]
Number of shares authorized for repurchase (in shares)									5,000,000
Number of shares repurchased (in shares)				0		0	0	2,030,000
Number of shares available for repurchase (in shares)				2,970,000
Cost of shares repurchased				$ 7,395	$ 4,088
Dividend declared and payable (in usd per share)			$ 0.41
Dividends paid (in usd per share)		$ 0.41		$ 0.41	$ 0.39
Subsequent Event
Class of Stock [Line Items]
Dividend declared and payable (in usd per share)	$ 0.43
Tax Withholding Obligations
Class of Stock [Line Items]
Number of shares repurchased (in shares)				133,000	78,000
Cost of shares repurchased				$ 7,395	$ 4,088

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