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Stockholders' Equity (Details) - USD ($)		3 Months Ended			6 Months Ended		12 Months Ended
	Jul. 20, 2018	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Apr. 20, 2018
Class of Stock [Line Items]
Cash dividends to common stockholders		$ 2,166,000		$ 1,660,000	$ 4,181,000	$ 3,315,000	$ 6,896,000
Cash dividends per common share (usd per share)		$ 0.31		$ 0.27	$ 0.60	$ 0.54
Shares withheld for tax withholding and exercise of options					19,863	5,651
Amount of shares withheld for tax withholding and exercise of options					$ 1,400,000	$ 385,000
Shares withheld for tax withholding and exercise of options, weighted average price (usd per share)					$ 70.34	$ 68.04
Reclassification from AOCI to retained earnings for stranded income tax effects					$ 0
Share Repurchase Program, amount approved to repurchase								$ 25,000,000.0
Stock repurchased, net of commissions		$ 104,000			$ 104,000
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%
Retained Earnings
Class of Stock [Line Items]
Cash dividends to common stockholders					$ 4,181,000		$ 6,896,000
Reclassification from AOCI to retained earnings for stranded income tax effects			$ 638,000		$ 638,000
Common Stock
Class of Stock [Line Items]
Stock repurchased, net of commissions (in shares)		1,398			1,398
Stock repurchased, net of commissions					$ 104,000
Subsequent Event
Class of Stock [Line Items]
Dividends declared per common share (usd per share)	$ 0.32

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