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Stockholders' Equity (Details) - USD ($)		3 Months Ended		12 Months Ended
	Apr. 20, 2018	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Class of Stock [Line Items]
Cash dividends to common stockholders		$ 2,015,000	$ 1,655,000	$ 6,896,000
Cash dividends per common share (usd per share)		$ 0.29	$ 0.27
Shares withheld for tax withholding and exercise of options		19,606	4,715
Amount of shares withheld for tax withholding and exercise of options		$ 1,400,000	$ 324,000
Shares withheld for tax withholding and exercise of options, weighted average price (usd per share)		$ 70.36	$ 68.80
Performance-based stock awards
Class of Stock [Line Items]
Vesting period of performance-based stock awards		3 years
Performance-based stock awards | Minimum
Class of Stock [Line Items]
Vesting percentage of performance-based awards		0.00%
Performance-based stock awards | Maximum
Class of Stock [Line Items]
Vesting percentage of performance-based awards		200.00%
Subsequent event
Class of Stock [Line Items]
Dividends declared per common share (usd per share)	$ 0.31
Share Repurchase Program, amount approved to repurchase	$ 25,000,000.0
Retained Earnings
Class of Stock [Line Items]
Cash dividends to common stockholders		$ 2,015,000		$ 6,896,000
Reclassification from AOCI to retained earnings for stranded income tax effects		$ 638,000

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